Abstract: In view of the effect on the sensing performance of multi-antenna multi-user in cognitive radio (CR) system by imperfect control channel, an optimal selection scheme of the number of collaboration users that minimizes the error detection probability of CR system is proposed, and the closed expression of the optimal number of collaboration users is demonstrated. The algorithm firstly analyzes the decision threshold of each antenna of CR user based on the minimal error detection probability. Secondly, “K” rank rule to merge the multi-antenna is adopted, and the optimal “K” value is deduced based on minimal error detection probability of each CR user. Thirdly, the optimal number of collaboration users is deduced based on minimal error detection probability of the system. The validity of theoretical derivation is proved through simulation, and the effect on CR system by the error probability of control channel is analyzed. From a detection performance perspective, the proposed scheme is advantageous comparing with conventional collaboration detection algorithms.