The expansion of response bandwidth is an important direction in the development of quantum well infrared photodetectors. Using quantum well material with the peak response wavelength at 10.55 μm, the diffraction grating structure in the 30 μm center distance quantum well infrared detector is optimized, and six different combinations of the mixed-period gratings are obtained by mixing three grating structures with the period of 2.80, 3.50, and 4.25 μm within a single photosensitive pixel. Photoresponse spectroscopy tests show that the response bandwidth of the mixed-period grating can be broadened from 1.20 μm to 1.91 μm by up to 60% compared to a single-period grating, while the blackbody responsivity decreases by only 12%.