Abstract: In pseudo-code ranging systems, the high rate and long code direct sequence spread spectrum (DSSS) can improve not only the communication performance but also the ranging accuracy. However, long code DSSS acquisition of weak signals is still faced with lots of challenges. Hence, an improved DSSS acquisition algorithm based on FFT was proposed,including the discontinuous search of code phase and parallel search of Doppler, the whole-bit coherent accumulation based on bit synchronization and the Tong judgment based on the frequency compensation of the time-frequency two-dimensional constraint. Both theoretical analysis and simulation results show that the proposed algorithm can substantially reduce the resource consumption and the probability of false alarm, shorten the acquisition time and eliminate the effects of data modulation under conditions of long time accumulation. In this paper such performance can be realized that the false alarm probability is less than 10^-6, the detecting probability is over 99.5% and the acquisition time is less than 5 s.