RWR (radar-warning receiver)/ESM (electronic warfare support measure) system usually uses traditional instantaneous frequency measure (IFM) technology to detect the frequency of signals in real time,but it can only estimate one frequency when it receives overlapping signals,and easily create frequency measurement errors.But in complex electromagnetic environment of the modern battle,pulses overlapping probability increase.So analyzing the influence of overlapping signals to IFM system is particularly important.Through the basic principle of IFM,analyzed each node response of IFM microwave phase discriminator in the case of double overlapping signals,built mathematical model of output U_I,U_Q and put forward a new algorithm—U_I,U_Q calculation algorithm that can estimate the frequency of double overlapping signals.Simulation results show that the algorithm can estimate the frequency of overlapping signals and keep advantages of original IFM.