The invention discloses a nuclear power station half-speed turbine primary frequency modulation control method and system. The method comprises the following steps that a target rotating speed value and an actually measured rotating speed value are received, and working frequency of a current half-speed turbine is calculated according to the target rotating speed value and the actually measured rotating speed value; the rated frequency of a primary frequency modulation dead band is arranged to be -1.25Hz-+1.5Hz in advance; whether a primary frequency modulation dead band switching signal is received is judged, and working frequency is outputted if the judgment result is yes; or working frequency and the rated frequency are compared, and zero or processing frequency after processing of the primary frequency modulation dead band is outputted; working frequency or processing frequency is received and frequency modulation processing is performed on working frequency or processing frequency, and a frequency contribution value used for controlling opening degree of a turbine steam inlet valve and working of a reactor is outputted; and opening degree of the turbine steam inlet valve and working of the reactor are controlled according to the frequency contribution value. With application of the nuclear power station half-speed turbine primary frequency modulation control method, the risk of overpower caused by power fluctuation of the reactor can be effectively reduced by expanding range of the rated frequency of the primary frequency modulation dead band.