In the long-wave infrared push-broom imaging system,compared with the residence-time of the system,the integration-time of the detector is only about 10% of the former because of strong background in our designing. To make full use of residence-time in push-broom imag-ing system and increase the SNR,the oversample-superposition method was applied in the designing. We described the method of oversample-superposition according to easy-saturated infrared detector,then analysed it's effection on system PTF and SNR. Furthermore,a successful design of oversample-superposition hardware and image capture software have been made. Some experiments were carried out to capture images of hand which indicate that this system performs well.