Abstract: The key for capturing the communication contents of a direct sequence spread spectrum (DSSS) communication system is how to estimate its pseudo-noise(PN) code in a non-cooperative communication condition. This paper proposes a time domain waveform processing algorithm based on Intrinsic Time-Scale Decomposition (ITD), which can accurately estimate the PN code under a condition of very low Signal/Noise Ratio(SNR). Taking advantage of high time-frequency resolution and convenience of real-time processing in ITD algorithm, it directly analyzes fluctuation characteristics of differential signal between cumulative amplitudes from carrier frequency point and the first up zero- crosssing frequency point to find the positions of polarity alternating points between two adjacent chips in a period of PN code and to reveal the PN Code，without necessity of guessing algebra structure, where coherent accumulation from many periods of PN code signals is employed to raise the SNR of the processed results. This algorithm requires low sampling accuracy, is insensitivity to carrier frequency estimation error and fits for all kinds of PN codes when compared with algorithms available. Computer simulation validates the feasibility of the proposed algorithm.