In view of the lacking of theory and method in determining missile system level testability indices, this research presents a testability indices determining method oriented to mission requirements. The missile mission is divided into four phases, such as storing, combat preparation, disposition and launch. Then the test requirement of each phase was proposed. Based on the performance requirements of store availability, combat preparation mission success rate, disposed combat readiness rate and launch mission success rate, some testability requirement models were built. The relationship between the performance requirements and testability parameters were given, and the testability parameters which were the key factors on the performance were selected. It defines an aggregative index to evaluate the missile mission performance, and then modifies the mission phases performance requirements to unify the testability indices which may vary in deferent missions. The credible testability indices are gained synthetically.