In this paper, a semi-blind multi-user detection (MUD) algorithm for CDMA systems is proposed where the codes of intra-cell users are known but the codes of inter-cell users are unknown. This semi-blind multi-user detection algorithm (SB-LSCM) is a joint MUD algorithm of decorrelating algorithm and Least Square Constant Modulus (LSCM) algorithm. The proposed approach fully utilizes all known users? information, performs non-blind detector to suppress the interferers within the cell, and exploits blind LSCM detector to suppress the remain interferences. The performance of the semi-blind LSCM detector is compared to that of the non-blind decorrelating detector and blind LSCM detector, and it is shown that the semi-blind detector can achieve a considerable performance improvement.