The present secondary safety measure in smart substation has low reliability due to the low safety and frequent protection blocking. On this basis, a safety measure based on interval interruption maintenance is proposed by analyzing the new function features of secondary equipment, as well as the advantage and disadvantage of the basic safety measures. The optimal scheme corresponding to the first maintenance and periodic maintenance is also determined. The secondary circuit will be cut off by disconnecting the terminal connecting piece or the cable conductor. Then the first defense line will be established by turning off the soft pressing board and breaking the digital channel. The second defense line is formed after pulling out the fiber or starting the smart substation maintenance mechanism. The first maintenances of 220 kV Sha-Ba substation and 220 kV Zha-Wo substation in Sichuan show that this scheme is simple, safe and available, which is of certain reference value for secondary safety measures in the smart substation maintenance.