A fiber-coupled laser diode (LD) end-pumped Tm:YAG laser operated under cryogenic condition is studied. The central wavelength of the fiber-coupled LD is 785 nm at 15 ℃, with fiber core diameter of 400 μm and numerical aperture of 0.22. The Tm:YAG crystal, with the dimension of 3 mm×3 mm×8 mm, is doped with Tm3+ with atomic fraction of 3%. Pulse tube cryocooler used to cool the crystal in the experiment is a new type of micro-cryocooler with the advantages of no vibration in the cold end, simple structure and long lifetime. The copper heat embedded with Tm:YAG crystal is attached to the cold head of the cryocooler and then fixed in a vacuum chamber to prevent frost. By adjusting the crystal temperature, the dependence of threshold and output power of Tm:YAG laser on crystal temperature in the range of 80～290 K is investigated. When the crystal temperature is 80 K, the highest continuous wave output power of 3.78 W at 2.013 μm under the pumping power of 9 W is achieved, corresponding to optical-optical conversion efficiency of 42%, and slope efficiency of 44.9%.