Abstract: The method of Metal Organic Chemical Vapor Deposition (MOCVD) was used to grow GaAs/Al0.3Ga0.7As quantum well material. Which is prepared for quantum well infrared photodetectors (QWIP). The two sample-devices have large surface area 300 m300 m. But its pressure welding area of inside electrode is 20 m20 m while that of the one outside is 80 m80 m. Refrigerating machine of liquid nitrogen was adopted to do dark current test under variable-temperature from 77 K to 300 K. The dark current was studied under different bias voltages. The results show that dark current curves is asymmetric under positive and negative bias voltage. The crystal structure is investigated by use of high -resolution transmission electron microscope (HRTEM) to determine the exact reson. Which shows that there is thread dislocation and nonuniformity in different degrees. The follows is known from above: It is the phase separation caused by the threading dislocation that leads to photoelectric performance variation essentially. At the same time, it is interfacial asymmetry between AlGaAs and GaAs in different growing orders and the doping element diffusion that intensifies asymmetry of dark current.