A graphene photodetector based on ultra-thin silicon waveguide at 1.55μm is proposed. By reducing the silicon core thickness, the fundamental TE waveguide mode is less confined and light-graphene interaction is enhanced. Benefiting from the ultrathin silicon waveguide and reflector structure, the graphene absorption coefficient reaches 0.36 dB/μm. A 10nm-thick CVD-grown hexagonal boron nitride is covered on the graphene to improve the device performance. With the help of metal-graphene-metal structure, the contact resistance is reduced dramatically. The devices have shown a responsivity of 1.4 mA/W at 0 V bias and 23.1 mA/W at 0.3 V bias with 0.24 mW input optical power. The measured 3-dB bandwidth is 17GHz under 0V bias voltage at 1550 nm.