Abstract: To solve absence problem of the method in determining the product-s testability level and find the difference between the practical testability situation and the requirements, the concepts of testability test and testability determine test was proposed. With the only consideration of fault detection rate (FDR), the method to establish fault character model (FCM) was given, which is based on the fault-function relation matrix and fault weightiness. Two-tuples FCM was illustrated and the flow to select sample based on two-tuples FCM in testability determine test was given. To simplify the workload and the complexity of the fault injection test, the equivalent relation between test samples were researched. The equivalent set and the equivalent injection sample (EIS) were defined. The relative method of result evaluation for FDR was researched. An example on a control system in some missile was illustrated, and the results show that the methods introduced are correct and feasible.