Abstract: PRBC-LFM signal is a combined modulation signal, which is widely used to design low probability intercept radar waveform. For the characteristic of the PRBC-LFM signal, a parameter estimation algorithm based on Fractional Fourier Transform is proposed. Firstly, search peak in two-dimensional time-frequency plane in fractional domain to get optimal rotating angular, estimating the chirp rate. Then, the FrFT with actually order is presented. The parameters, including bandwidth, initial frequency, code time width, carrier wave frequency of the signal, is estimated through the relation between the peaks’ distance and the parameters of the frequency modulation signal. Finally, the linear frequency modulation signal can be reconstructed and multiplied with the conjugated signal in every code to remove the carrier wave. The coding rule of the biphase coded signal can be identified through division the sign of the signal’s amplitude. Simulation results show that the algorithm in this paper can estimate the parameters of the PRBC-LFM signal effectively at low SNR.