Currently, the conversion efficiency of Cu2ZnSn(S, Se)(4) (CZTSSe) thin film solar cell is mainly limited by its low open-circuit voltage. In this work, the performances of CZTSSe thin film solar cell were calculated by the simulation program AFORS-HET v2.5 to reveal some useful ways to enhance the open-circuit voltage. First, several factors affecting the open-circuit voltage were analyzed. The open-circuit voltage of CZTSSe solar cell was significantly improved by reducing the defect states, interface states, parasitic resistance effect, and formation of MoS 2 layer, and adjusting the hole concentration or band gap of CZTSSe absorber. Then, the band gap gradient was introduced into the CZTSSe absorber to extend the built-in electric field and enhance absorptions, leading to benign impacts on the open-circuit voltage of CZTSSe solar cell. Finally, a back surface field was considered for the CZTSSe solar cell. The effects of CZTSSe and amorphous silicon back surface fields depended on the defect states. The advantage of back surface field was prominent when the defect density was low. It was necessary to reduce the defects before considering a back surface field. In the calculations, the best CZTSSe solar cell had an open-circuit voltage deficit of only 0.134 V. The numerical analyses in this work provide some methods to improve the open-circuit voltage and conversion efficiency CZTSSe thin film solar cell. The proposed methods offer guidance for experimental works.