It is customary for a substation automation system to use the GPS timed device to protect the time synchronization between IEDs， the time-frequency mode being generally adopted to test the accuracy of the output signal. This paper proposes a new testing method as a supplement and improvement with such important features as the application of the controllable GPS real-time timing pulse， the control of the timing mark pulse output at the designated moment， and the action of the switch， testing the accuracy and correctness of the GPS timed device output signal. That is， the new method can test not only the real-time accuracy of the GPS timed device output signal， but also the correctness of the time encoding information carried by the output signal.