The grating optical coupling method is commonly used for detecting the normal incident light with a quantum well photodetector. In this paper, the distribution of electromagnetic field in one-dimensional metal grating terahertz quantum well photodetectors is studied with modal method, and the average light intensity in the active region is obtained. The result shows that, the light intensity in the device can be optimized when the period of the grating is comparable with the wavelength of the terahertz wave in the sample and a proper filling factor is chosen according to the device structure, which will improve the responsivity of the device.