The invention relates to a speed keeping device for a nuclear power water-feeding pump set and a nuclear power water-feeding pump set system. The speed keeping device comprises a hydraulic coupler, alockup valve, a controller and the like. Due to the fact that the retaining valve is arranged on an external oil supply pipeline of the hydraulic coupler, the hydraulic coupler in the nuclear power water-feeding pump set has a retention function, when the controller judges that a nuclear power unit has abnormal working conditions such as faults of a spoon tube position sensor in the hydraulic coupler or faults of the controller, the controller can control the retaining valve to cut off an oil path so that the position of a spoon tube in the hydraulic coupler can be kept, then the output rotating speed of the hydraulic coupler is kept unchanged, and the outward water supply amount of the nuclear power water-feeding pump set is kept unchanged. The nuclear power water-feeding pump set systemadopting the speed keeping device has the operational reliability in a fault state, so that that the situations that when the faults occur, the large fluctuation of the output rotating speed is causeddue to the fact that the position of the spoon tube automatically returns to the zero position, and then the unit transient sate is caused are effectively avoided, and enough barrier-removing intervention time is also reserved for workers.