In this paper, the effects of Ni and Au back electrode on the performance of CdTe solar cells and the mechanisms have been studied by experiment. Among the back electrodes of Ni, Ni/Au, Au/Ni and Au film, only when the Ni film contacts the ZnTe/ZnTe:Cu complex layers, does the conversion efficiency increase with the short current increasing. So it is probable that the conversion efficiencies of cells will be increased by 4% at least, and the cost in large scale manufacturing will be cut down. By analyzing the dark C-V/I-V cures and the XPS using different back electrode films, it has been found the concentration and depth of Ni diffusion in the ZnTe/ZnTe:Cu back contact layer are greater than that of Au. What's more, most Nix+ combine with the abundant Te ions in ZnTe/ZnTe:Cu back contact layer to form the NixTe. So the diffusion concentration is increased, which makes the characters of CdTe improved.