Abstract: To ensure the safe and reliable operation of the space-arm when it working in the space, the space-arm should be locked reliably during launching. The optimal number and position of the locking spots should be determined when consider locking plan. The finite element analysis method was employed to accomplish modal analysis of space-arm under different constraints state. Modal analysis was employed in simplified model of the space-arm. The relative larger displacement of the part can be identified form mode shapes which can help to determine the key locking spots. Moreover, the auxiliary locking points will be selected according to the characteristics of the arm. Thus, all the locking spots of space-arm with a series of multi-joints were selected primary. Under such locking condition, the dynamic characteristics of the space-arm will be improved increasingly. Depending on the mass characteristics of the support mechanism, its model is simplified into springs and reasonable constraint equations were established to simulate their properties. This study provides an effective theoretical basis for dynamic design and optimization design of the space-arm and the lock-unlock mechanism.