This paper discusses the multi-signal flow graph model and the testability analysis method based on this model. The modeling process of the multi-signal model, the dependency matrix generation method of the failure-test and the details of the testability analysis are illustrated by use of several examples. The results got in the testability analysis are also validated. It is demonstrated that the testability analysis method based on multi-signal model is easy to be implemented in computer-aided analysis, which helps to develop the testability analysis software tool in future.