The invention belongs to the technical field of nuclear reactor protection, and particularly relates to a designing method for an external output signal test loop of a reactor protection system. The method can eliminate a wrong action risk, caused by an external output connection signal test, of third-party system equipment. Test signal time interval is prolonged, elimination of a previous test feedback signal before each test signal is tested is determined, in this way, the wrong action risk, caused by the external output connection signal test of the full-digitalization reactor protection system, of the third-party system equipment is eliminated fundamentally, shutdown reacting-stopping risks generated by regular testing during operation of unit power are effectively prevented, and the reliability of the full-digitalization reactor protection system is effectively improved.