A Quantum Well Infrared Photodetector Light Emitting Diode (QWIP-LED) is a new infrared up-converter. When combining with a Charge Coupled Device (CCD), it can be used for mid-wave infrared imaging detection. Compared with the conventional infrared imaging devices, QWIP-LEDs have a remarkable advantage of low cost. However, because of the low light extraction efficiency (~2%), their application is limited. To improve the light extraction efficiency of the QWIP-LED, a Photonic Crystal Slab (PCS) is incorporated into the surface of the LED. Then, a Finite Difference Time Domain (FDTD) method is used to calculate the improved light extraction efficiency, so as to optimize the structure parameters of the PCS. The simulation result shows that by using an optimized PCS, the light extraction efficiency of the QWIP-LED can be increased by a factor of 2.32. Finally, the mechanism for the PCS to increase light extraction efficiency is analyzed according to the band theory and equivalent medium theory.