As one of the earliest application of graphene, photodetector is widely considered to be promising in the future during various graphene based devices. Thanks to the high photoelectric conversion efficiency of graphene/Si heterojunction and the convenient assembly technology, graphene/Si photodetectors have attracted more research interests. Herein, we firstly introduce the background of graphene/Si photodetectors and then analyze the mechanisms in details. Based on several typical photodetector structures, the performances of graphene/Si photodetectors are discussed, and the expectation of graphene/Si photodetector is proposed.