640×512 Quantum Well Infrared Photo-detector(QWIP) long wavelength infrared(LWIR) focal plane arrays(FPA) were made by using GaAs/AlGaAs quantum well structure. The response spectra were at 10.55 μm. Integrated with rotation sterling cooler which gave a temperature of 50 K, the FPA was measured to find the results that NETD was about 22.5 mK. The FPA assembly kept good performance after the switch-on-off cycling test and thermal vacuum test. The illumination non-uniformity of the focal plane with a cold shield was calculated with a numerical method. The results were compared with the approximate analytic method. It is shown that numerical method should be used for small F-numbers. Thus, the measured non-uniformity is believed to be dominated by the illumination non-uniformity. By using the MEEP FDTD software, calculation about the near-field photo-coupled electrical-field energy were designed and results show that current QWIP structure parameters are near optimized in term of optical diffraction.