A focusing device includes a vertical-synchronization-signal generator generating a vertical-synchronization-signal sequence including vertical synchronization signals, an excitation-sequence generator generating an excitation sequence according to the vertical-synchronization-signal sequence and applying excitation according to the excitation sequence, and a driving module driving a lens of an imaging system according to the excitation sequence. Vertical-synchronization-signal times are formed between two adjacent vertical synchronization signals. A first vertical-synchronization-signal time and a last vertical-synchronization-signal time are formed at a beginning and an end of the vertical-synchronization-signal sequence respectively. Each vertical-synchronization-signal time includes a sub time and an exposure time arranged sequentially. A preceding excitation is only applied in the sub time of the first vertical-synchronization-signal time, a succeeding excitation is only applied in sub time of the last vertical-synchronization-signal time and a phase-fixed excitation is applied in each exposure time of each vertical-synchronization-signal time except the exposure time of the last vertical-synchronization-signal time.