In this paper, Cu2ZnSnSe4(CZTSe) thin films were prepared by solution method. A small amount of Ge was added into the solution and the effect of Ge incorporation on the properties of CZTSe thin films and devices was investigated. For comparison, two sets of CZTSe thin films and corresponding solar cells, namely CZTSe without Ge and CZTSe with a small amount of Ge［Cu2Zn(Sn, Ge)Se4, CZTGSe］were prepared. The crystal structure, phase purity, surface morphology, and carrier concentration of the prepared thin films, as well as the electrical properties of the complete devices were characterized and analyzed by X-ray diffractometer (XRD), Raman spectra, scanning electron microscopy (SEM), Hall measure system, current voltage (J-V) curve, and external quantum efficiency (EQE) tests, respectively. The results demonstrate that Ge incorporation into the CZTSe film promotes the crystallization mainly due to the formation of Ge-Se liquid flux during the selenization process, reduces the number of grain boundaries and the minority carriers recombination at the grain boundaries and enhances the carrier lifetime. In addition, partial substitution of Ge for Sn reduces the density of defect states associated with Sn, increases the energy band gap and open circuit voltage, improves the series and parallel resistance, and thus increases the fill factor. Finally, the prepared CZTGSe thin film solar cells reach a photoelectric conversion efficiency of 8.83% with open circuit voltage of 513.2 mV, short circuit current of 27.47 mA/cm2 and fill factor of 62.68%.