The invention discloses a frequency measurement device of a hydroelectric generating set. According to the frequency measurement device of the hydroelectric generating set, a frequency divider and a clock pulse signal are used for processing voltage signals of the set and a power grid, frequency signals of the set and the power grid can be measured accurately, the measurement result is stable, signals under multiple frequency division states of the set and the power grid can be measured at the same time, phase difference signals of the waveform of a generator and the waveform of the power grid can be measured at the same time, measurement of the frequency of the set and the power grid is more accurate, and synchronization and grid connection between the set and the power grid are achieved.