Graphene has a low absorption rate of light. By combining with an optical resonant cavity to restrict the light field, the absorption rate of the graphene detector element can be effectively improved. Based on the theory of electromagnetic field transmission, the optical field distribution in the bilayer graphene optical resonator is deduced, a mathematical model for the transmission matrix of the resonant enhanced photodetector is established, and a resonant enhanced photodetector for the Bernal-Stacked bilayer graphene is established. The structural parameters are numerically calculated and the detector performance is analyzed. The results show that, the optical absorption rate of bilayer graphene reaches up to 96.78% for a 1.06 μm wavelength resonant enhanced photodetector, which greatly improves the detection ability of weak optical signals.