Ag<sub>2</sub>ZnSnSe<sub>4</sub> is an n-type semiconductor with a suitable bandgap of 1.4 eV. In the present study, a graphene/Ag<sub>2</sub>ZnSnSe<sub>4</sub> induced p-n junction thin film solar cell is proposed and the physical mechanism and performance influencing factors of the solar cell are simulated and analyzed by using the wxAMPS software. The simulation results show that when a high work function graphene contacts an n-type Ag<sub>2</sub>ZnSnSe<sub>4</sub> semiconductor, the energy band of the Ag<sub>2</sub>ZnSnSe<sub>4</sub> absorber layer bends upward, meanwhile a p-type Ag<sub>2</sub>ZnSnSe<sub>4</sub> inversion layer is induced on the surface of n-type Ag<sub>2</sub>ZnSnSe<sub>4</sub>, therefore the p-type Ag<sub>2</sub>ZnSnSe<sub>4</sub> and n-type Ag<sub>2</sub>ZnSnSe<sub>4</sub> form an induced p-n homojunction. It is found that the work function of graphene and back contact significantly influence the photogenerated carrier separation, transportation and collection. The graphene work function should be 5.5 eV and the work function of back contact should not be greater than 4.4 eV, which is beneficial to the improving of the device performance. The doping concentration of Ag<sub>2</sub>ZnSnSe<sub>4</sub> absorber mainly affects the short-circuit current of the device, however, the defect density of Ag<sub>2</sub>ZnSnSe<sub>4</sub> absorber affects the whole device performance. When the work function of graphene and back contact are 5.5 eV and 3.8 eV, the doping concentration and defect density of Ag<sub>2</sub>ZnSnSe<sub>4</sub> absorber are 10<sup>16</sup> cm<sup>–3</sup> and 10<sup>14</sup> cm<sup>–33</sup>, respectively, the conversion efficiency of the graphene/Ag<sub>2</sub>ZnSnSe<sub>4</sub> induced p-n junction thin film solar cell can reach 23.42%. These simulation results provide the idea and physical explanation for designing a novel type of solar cell with high efficiency and low cost.