A CuIn_1-xGa_xSe₂ (CIGS) thin film solar cell model with MoSe₂ transition layer was established, using SCAPS-1D software. The influence of MoSe₂ interface layer formed between absorption layer CIGS and the back contact Mo on the solar cell performance was investigated. By changing the doping concentration, thickness and bandgap of MoSe₂ layer, it is found that the MoSe₂ and the variation of parameters have a significant effect on the electrical characteristics and photovoltaic parameters of CIGS thin film solar cells. Based on the energy band, the interfaces of Mo/MoSe₂ and MoSe₂/CIGS are analyzed. It is considered that Mo/MoSe₂ is a Schottky contact, MoSe₂/CIGS is an ohmic contact. When suitable parameters of MoSe₂ layer are formed into the interface, it will provide a new path for designing CIGS solar cells with thinner absorption layer.