A large scale, 640×512 pixel, readout integrated circuit (ROIC) with snapshot integration mode for cooled IRFPA has been fabricated using a standard 0.5μm double-poly double-metal (DPDM) CMOS process. The pixel pitch is 25μm and an improved direct injection (DI) input circuit is used. The integration control mode is integrate-then-read. In this paper, the circuit′s structure and operation principle are analyzed first. Then, the transfer characteristic of the input unit cell is simulated and the testing results of an IRFPA interconnected with this ROIC are given at last. The results show that this ROIC is feasible for large format and small pixel IRFPA.