It is widely accepted that graded buffer layer between the p-layer and i-layer increase the efficiency of amorphous silicon solar cells. The open-circuit voltage (Voc), short current density (Jsc) and fill factor (FF) of the thin film solar cell are obviously increased. In the present study, hydrogenated amorphous silicon (a-Si:H) thin film solar cells have been fabricated by 27.12 MHz plasma enhanced chemical vapor deposition (PECVD). We discussed the three conditions at the p/i interface without buffer layer, buffer layer and graded buffer layer of thin film solar cells by TCAD software. The influences of the performance of the solar cell with the different buffer layer are investigated. The cell with graded buffer layer has higher efficiency compared with the cells without buffer layer and buffer layer. The graded buffer layer enhances the conversion efficiency of the solar cell by improving Voc and FF. It could be attributed to a reduction of interface recombination rate near the junction. The best performance of conversion efficiency (η)=8.57% (Voc=0.81 V, Jsc=15.46 mA/cm2, FF=68%) of the amorphous silicon thin film solar cell was achieved.