Abstract: This paper studies the problem of estimation of the spread-spectrum sequence for low SNR synchronous multi-user long-code direct sequence spread spectrum (DSSS) signals.An estimation approach of spread-spectrum sequence based on a nested iterative least squares with projection (NILSP) algorithm is presented.First, The synchronous multi-user long-code DSSS signals is constructed as the short-code ones with missing data.Then the mathematical analysis for the short-code ones with missing data is done based on the theory of maximum likelihood estimation (MLE) to establish a mathematical model of spread-spectrum sequence.After that, the spread-spectrum sequence is estimated by a NILSP algorithm.The theoretical analysis and simulations show that, the proposed algorithm always has a good performance for estimation of multi-users' (more than 10 users) spread-spectrum sequence in low-SNR (lower than -10 dB) scenarios, and under the same conditions, the longer the data size, the better the performance.