In this work,the enhancement effects of the photonic-crystal-based light coupling structure on the absorption efficiency of quantum well infrared photodetectors (QWIPs) are studie d.By using an electromagnetic model of absorption efficiency and finite element method,we analyze the field d istributions of the resonant modes in the photonic crystal slab,and the relations between the absorption efficiency and the structure parameters of the photonic crystal.Furthermore,by comprehensive optimization,a QWIP absorption efficiency about 88% can be obtained based on the resonant enhancement effect of photonic crystal structures .