A clock generating device is disclosed. The clock generating device includes a clock generating unit, for counting a synchronization period of a synchronization signal, generating a first interrupt signal according to the synchronization signal, generating a pulse-width modulation signal according a control signal, counting a phase difference between the synchronization signal and the pulse-width modulation signal, and generating a second interrupt signal according to the pulse-width modulation signal; and a computing unit, for acquiring the synchronization period according to the first interrupt signal, acquiring the phase difference according to the second interrupt signal, and adjusting the control signal according to the synchronization period, a modulation period of the pulse-width modulation signal and the phase difference.