The custom windowing is an important technique for the observation of areas of interest and the detection of special areas in large-scale infrared focal plane applications by reconstructing image resolution to improve the readout frame rate. Based on the semi-custom design flow, a readout integrated circuit (ROIC) digital module with custom windowing function is proposedin this paper, which can realize the functions of integral time regulation, working mode switching, random windowing and anti-overflow under the control of five external input signals, and has the advantages of easy expansion, simple control and flexible operation. Aiming at the risk of race and hazard in the traditional decoding circuit, the design of row-level pulse width adjustable selection signal and the solution of column-level multi-port readout are proposedto further improve the reliability of the circuit. The simulation results show that the whole design can properly implement the function of custom windowing normally, and is suitable for large-scale infrared focal plane array.