Quantum well Infrared photodetector (QWIP) has high impedance, fast response time, long integration time, low power consumption and so on, special advantages in the area of Very Long Wavelength infrared (VLWIR) Focal Plane Array (FPA) applications. Readout integrated circuit (ROIC) is one of the core parts of FPA. On this paper, A Capacitive Feedback Transimpedance Amplifier (CTIA) CMOS ROIC was adopted for QWIP VLWIR FPA. Sensible illustrations with simulated result are showed. The simulated discharging waveforms of integration capacitor Cin, at 55K, 77K, and 150K respectively is given. At last, we more analyze operation characteristics of CMOS in cryogenic temperature.