As the construction of the digital substation, intelligent equipments such as electronic transformer is widely used. In the process of electronic transformer’s converting analog signal into digital signal, the integral cycle delay phenomenon will follow. In this paper, the reason of the whole cycle delay of electronic transformer was analyzed. The detection principle and method for such phenomenon was also proposed. According to the concept of power source triggered by a pulse and the first-frequency-output power source, related testing equipment was designed. And the equipment can help us to know if there is an integral cycle delay for the electronic transformer in the most intuitive and most accurate way.