Diffraction grating is indispensable for quantum well infrared photodetectors(QWIP) in order to improve optical coupling because of the limitation of quantum selection rules. A numerical approach based on finite difference time domain(FDTD) method was presented for investigating the diffraction effect of a metal grating fabricated in QWIP. Simulation results demonstrate that such a complicated structure can be optimized by electromagnetic analysis based on the FDTD method. Detailed field distribution inside QWIP can be obtained for estimating the optical coupling efficiency of the grating.