Locking mechanism of a deployable mast was modified. On the base of working principle, the mechanism’s failure mode was analyzed and a new model of reliability was constructed. In the process of calculating reliability index, the factors such as size, assembly error, and space environment influence were taken into consideration, and the method of MFOSM (Mean First Order Second Moment) was applied. Based on the actual size of the mechanism, locking reliabilities at both room temperature and space limit temperature were calculated. The main factors that influence the reliability were analyzed and the improvement was put forward. The result shows that the new model has a high reliability.