The invention relates to a speed governor control method for power generation working condition of a water-turbine generator set. The speed governor control method comprises the following steps: judging the state of an outlet switch of the generator set; stopping a procedure if the outlet switch is in switched off state, or judging another criterion if the outlet switch is in switched on state; receiving a rotating speed signal of the generator set via a transmitter; determining that the generator set is in a primary frequency modulation rotational speed dead zone when the rotating speed ranges from 100% to 100.13%; carrying out primary frequency modulation of the speed governor when the rotational speed is higher than 100.13%, wherein the time of frequency modulation is T; superimposing primary frequency modulation additional power on set power of the generator set; sending an action value to a proportional servo valve driver from a speed governor PLC; adjusting a proportional servo valve via the proportional servo valve driver to control a main distributing valve; carrying out primary frequency modulation when the time reaches up to T; withdrawing. The speed governor control method has the advantages that the sampling accuracy of frequency modulation data is improved, the reliability and the correctness of the primary frequency modulation of the generator set are guaranteed, generator set breakdown is reduced, reliable running of the generator set is guaranteed, and the service life of the generator set is prolonged.