Relay board is taken as the output driving part of the reactor protection system in one nuclear power plant. The third periodic test of the reactor protection system is done during factory test, and the phenomenon is discovered that the relay board issues trigger signals last about 20ms when the bypass of the safety trigger cabinet switches from bypass position to normal position. The influence of the transient trigger signal on the normal operation of the nuclear power unit is analyzed. The third periodic test principle of protection system is analyzed combined with the internal structure of relay board,and the relay jumping as the reason of false triggering is concluded. The problem is solved by adding the solid state timer without changing the original structure of relay board. It provides a reference for the similar design of the following reactor protection system.