Abstract: Aiming at the difficult problem of periodic detection and estimation of the pseudo noise(PN) sequence in the DS-SS signals with residual carrier,a novel detection method is developed.Firstly,the power spectrum of weak DS-SS signals with residual carrier is calculated,and then the second power spectrum is calculated once again by taking the first one as an input signal.Some regular tip-pulses appear at the integer times of period of the PN sequence while obtaining the second power spectrum.Thus,the period of PN sequence can be detected and estimated by measuring the interval of the regular tip-pules.The computer simulation shows that the proposed method can work well on the lower signal to noise ratio input signals with larger residual carrier.