The invention discloses an output control method and an output control system of a nuclear power plant reactor protection system, which belong to the field of nuclear power technologies. The output control method comprises the steps of: S1, carrying out transmission processing on an input signal of a nuclear power plant protection system so as to generate an output signal at an output end of the nuclear power plant protection system, and carrying out signal quality judgment on the output signal to obtain a judgment result; S2, retrieving a preset default value to perform numerical value replacement on the output signal when the judgment result indicates that the signal quality of the output signal is ''poor''; S3, and carrying out logic operation and subsequent control and display according to the default value. According to the output control method and the output control system, an existing output control method of the nuclear power plant reactor protection system is optimized, thus the optimized output control method can satisfy functional requirements of the default value of the nuclear power plant reactor protection system, and can reduce the influence of implementation of functions of the default value on other functions and performances of the system.