The distribution of electromagnetic field of 2D metal grating in the Long-Wavelength Quantum Well Infrared Photodetectors (LW-QWIPs) was studied by Finite Time Domain Method (FDTD). The result shows that making good use of the surface plasmons can remarkably improve the optical coupling efficiency in the LW-QWIPs. It also shows that using a metal grating with the ratio of the hole diameter to the grating period ranged from 0.8 to 0.9, optimal optical coupling can be achieved.