A kind of amorphous/nanocrytalline Si (a-Si:H/nc-Si:H) PIN type tandem solar cell of TCO/a-SiC:H(p1)/buffer/a-Si:H(i1)/nc-Si:H(n1)/tunnel junction (TJ)/nc-Si:H(p2)/ nc-Si:H(i2)/ a-Si:H(n2)/Al was optimized with numerical method. The high conductivity, high optical transmittance and wider band-gap material was selected as window layer in the cell. The buffer layer between p1 and i1 layers and the tunnel junction were designed, respectively. The thickness and band-gap of intrinsic layers in sub-cells were adjusted separately. The simulation results indicate that absorption solar spectrum by the designed cell can be expanded towards the long wave direction. The light-induced recession of present cell was restrained while the stability was improved.