The method is for identifying a signal and its image signal input to the heterodyne frequency sweep system. Comparing to a traditional heterodyne frequency sweep technology, a mixing module with a positive and negative sign filters (PNS filters) is introduced, and the detection and identification module is added, and the typical low-pass filter in the input module is removed. According to the timing order sequence of the signal output from the intermediate frequency (IF) filter and PNS filters, a signal and its image signal can be identified. The bandwidth of the heterodyne frequency sweep system is not limited by the cut-off frequency of the low-pass filter (LPF) of the traditional heterodyne frequency sweep system.