According to the structure、output signal characteristics and working demands of 320×256 longwave area array infrared detector, a focal plane circuit is introduced. The designed circuit consists of bias voltage engender circuit, signal preprocessing circuit and sequence driving circuit. The bias voltage engender circuit not only achieves the design of fixed bias voltages, but also achieves adjustable bias voltage’s design, which enhances the flexibility of this circuit. In this paper, the designed circuit is simulated and tested after the detailed introduction of every part’s designing scheme and completed the plating of the circuit. Simulation and test results show that the circuit could drive this infrared detector to work normally and could achieve the output analog signal’s preprocessing. This design idea of focal plane circuit for 320×256 longwave area array infrared detector is feasible and meaningful.