Abstract: Dual-band quantum well infrared photodetectors(QWIP) have been proved to be a kind of important third-generation infrared photodetectors. However, there are few systematic calculations and optimization design on the structure of its active regions and coupling grating. Based on envelope function approximation, transfer-matrix method(TMM) and classical grating theory, a MWIR-LWIR dual-band QWIP was designed, and detailed calculations on the structural parameters of multi-period active regions and 2-D coupling grating was presented. Lattice matched GaAs/AlGaAs MQW and strained InGaAs/GaAs/AlGaAs mini-band SLs were adopted as LWIR and MWIR active regions respectively, and the corresponding peak absorption wavelengths were 8.5 m and 4.5 m. Moreover, the type of electron transition was designed as B-QB to decrease the dark current and increase the detectivity. Besides, through optimized design, the dual-band incident IR radiations of LWIR and MWIR can be coupled effectively by a 2-D diffraction grating with single period.