Abstract: Based on the analysis of the system model, a user number estimation algorithm based on the first-order rate of eigenvalue is proposed for DS_CDMA system. The algorithm solves the problem that the traditional information theory method is not applicable to the estimation of the number of DS_CDMA system users by asynchronous demodulation processing. Then, the out-of-step points are estimated by the displacement correlation method, and the data matrix is extracted. Finally, the eigenvalues are extracted. According to the similarity of noise, the information theory algorithm based on the firstorder change rate of eigenvalue is used to estimate the number of users. The simulation results show that the algorithm proposed in this paper can correctly estimate the number of users in both synchronous and asynchronous systems when the SNR is -6 dB. Compared with other algorithms, the estimation accuracy is improved by 15%~20%.