Abstract: In order to improve the sensitivity and detectivity of the long-wavelength quantum well infrared photodetector (QWIP) ,the surface plasmon was adopted to enhance the optical coupling efficience of two-dimensional grating .The modulation effects of the 2-D metal film grating parameters in long-wavelength QWIP on the incident light were analyzed by using the 3-D finite-difference time-domain (FDTD) algorithm based on surface plasmon. Calculation results shown that the electric field along the Z direction reaches its maximum in the X-Y plane when the grating parameters were taken as P=2.8 m , D=1.4 m and L=0.04 m for 8 m incident light.