The invention relates to a preparation method of a flexible amorphous silicon thin film solar cell, and the selected flexible substrate is a polyimide film (PI film). The main process steps are as follows: (1) cleaning the flexible substrate (PI film); (2) pre-drying the PI film; (3) preparing an Al back electrode; (4) preparing a ZnO thin film buffer layer; (5) preparing an NIP amorphous siliconthin film by utilizing PECVD; (6) and preparing an Al front electrode. As the preparation method adopts the pre-drying process of the PI film and the ZnO thin film buffer layer, the photoelectric conversion efficiency of the cell is improved. The flexible cell has excellent flexibility and can be curled arbitrarily. Simultaneously, the preparation process is simple, and the large-scale productioncan be realized.