In order to solve the problem of low light absorption of silicon nanowire photodetector, we construct a hexagonal silicon nanowire structure of photodetector. We cover the structure with zero-bandgap graphene and add Au grating at the same time, and finally use COMSOL software to model and analyze the structure. It is shown that in the optical band range between 0.5 μm and 1.5 μm, the graphene coverage and the Au grating can effectively enhance the light absorption performance of the device. What is more, the thickness of both graphene and Au grating has an effect on the performance of the device.