The extensive application of low probability of intercept (LPI) radars caused most of the interception of radar signals with low SNR. The performance of the common parameter estimation algorithms of radar signals will be inevitably corrupted in low SNR. To estimate the parameters of radar signals combined pseudo-random binary phase code (PRBC) and linear frequency modulation (LFM) in low SNR, a novel method was proposed based on the poly-phase filter bank and high-order cumulant.Firstly,it realizes the uniformity signals in the frequency domain rapidly by the poly-phase filter bank.Then the Gaussian noise can be suppressed by using the short-time estimate of the third-order cumulant on each narrow frequency band.The estimation of the whole time-frequency matrixes can be obtained with envelope detection.Finally,the chirp rate, code-width and code-length of compound signal can be estimated by Radon transform of the Time-Frequency images. At the same time,the carrier frequency, band width, start-stop frequency can be estimated by extract the frequency curves. Simulation results show that all parameters of the compound signal can be evaluated precisely when SNR is more than -11dB.