Thermoelectric (TE) materials as one of clean energy materials can realize the direct conversion between heat and electricity. Flexible TE films have attracted recent attention because they can be used to fabricate flexible TE generators (f-TEGs) to replace batteries that need to be charged or replaced frequently to power rapidly developing wearable electronic devices. Bi2Te3-based films exhibit the optimum TE performance at room temperature (RT), but Bi2Te3-based films are easy to oxidize, and Te is expensive, rare, and toxic, so suitable alternatives need to be explored. Silver selenide (Ag2Se) is a narrow bandgap semiconductor and has a superior TE performance at RT. Furthermore, compared with element Te, element Se is less toxic and more abundant. However, the TE properties of Ag2Se are still lower than that of Bi2Te3. This review summarized the mechanism of improving the TE properties and flexibility of Ag2Se by functional unit order strategy, and provided a route to enhance the TE properties and flexibility of Ag2Se. Typical strategies for improving the zT values of Ag2Se-based films include the introduction of secondary phase, defect engineering, stoichiometry manipulation, etc.. However, charge carrier and phonon transport in TE material are strongly coupled with each other, seriously restricting the improvement of TE performance. The functional unit order is an effective strategy to optimize the performance of materials. Functional units with different roles constructed orderly in materials can enhance a synergistic effect between functional units and thus improve the performance. For Ag2Se-based flexible TE films, the effect of functional unit ordering and the synergetic effect between functional units on the electronic, phonon transport, and flexibility were discussed. The S-doped Ag2Se film shows a power factor ~954 μW·m-1·K-2 at 300 K and superior flexibility (94.4% of the original electrical conductivity was preserved after bending for 2 000 times around a rod with a radius of 4 mm). The S-doped Ag2Se film consists of a special “core-shell” microstructure, namely, the “core” is an S-doped Ag2Se phase and the “shell” (thickness of 15 nm) is Se-doped Ag2S and amorphous S mixed phase. The formation of the “core-shell” microstructure is mainly due to the different affinities between Ag and Se and Ag and S. The “core” acts as TE functional units because of the well-preserved conductive paths formed by well-crystallized S-doped Ag2Se grains, thus maintaining a high electrical conductivity of the film. The “shell” acts as a flexible functional unit to enhance the flexibility of the film mainly due to the inherently flexible amorphous S phase and the ductile Se-doped Ag2S. Moreover, the thermal conductivity of the film is low because of the strengthened phonon scattering resulting from the hetero-interfaces and the intrinsically low thermal conductivity of the amorphous S phase and the Se-doped Ag2S nanograins. The Ag2Se/PVP composite film has a high power factor of ~1 910 μW·m-1·K-2 and a superior flexibility at 300 K. Most Ag2Se grains with coherent interfaces in the Ag2Se/PVP composite film indicate that the Ag2Se TE functional unit is constructed, leading to a high electrical conductivity. The formation of the unique microstructure is related to the special sintering mechanism and the effect of PVP. The superior flexibility (i.e., 98% of the original electrical conductivity is preserved after bending for 1 000 times around a rod with a radius of 4 mm) is due to the good adhesion and viscosity of PVP acting as a flexible functional unit. The composite film has a pretty low thermal conductivity because of PVP with an extremely low intrinsic thermal conductivity and it contains numerous nano-sized- to micron-sized-pores and Ag2Se/PVP hetero-interfaces. The Ag2Se/Se/PPy composite film has an exceptionally high power factor of ~2 240 μW·m-1·K-2 and excellent flexibility at 300 K. The Ag2Se/Se/PPy composite film has a superior crystallinity of Ag2Se grains and continuous grain boundaries without defects, indicating that the Ag2Se TE functional unit is constructed, leading to a high electrical conductivity. There exist energy-filtering effects at the heterointerfaces of Ag2Se/Se and Ag2Se/PPy in the film, leading to a high Seebeck coefficient. The formation of the unique microstructure is related to the special sintering mechanism. The superior flexibility (i.e., 92.5% of the original electrical conductivity is preserved after bending for 1 500 times around a rod with a radius of 4 mm) is due to the good adhesion and viscosity of PPy acting as a flexible functional unit. Moreover, the nano-sized to submicron-sized pores, and the Ag2Se/PPy and Ag2Se/Se heterointerfaces in the composite film can scatter short- to long-wavelength phonons because PPy has an intrinsic low thermal conductivity, the composite film has a low thermal conductivity. Summary and prospects The functional unit order is an effective strategy to optimize the performance of materials. For Ag2Se-based TE films, the TE functional units ordering (such as coherent adjacent Ag2Se grains, good crystalline qualities, continuous grain boundaries, and core-shell nanostructure) and flexible functional units were constructed in Ag2Se-based films. The effective regulation of electron and phonon transport and flexibility was realized due to the synergistic effect between different functional units, thus increasing the electrical conductivity, reducing the thermal conductivity, and improving the flexibility. The more work are needed to verify whether the relevant strategies are universal.