generating a setting modulation matrix and loading the modulation matrix to an optical modulator, receiving an optical signal emitted or transmitted by a target object at a current focusing position, and modulating, by the modulation matrix, the optical signal;

acquiring a geometric moment value of the target object at the current focusing position according to the modulated optical signal;

calculating a central moment, that corresponds to the target object at the current focusing position, according to the geometric moment value; and

serving a focusing position obtained by refocusing as the current focusing position, returning the optical signal of the target object that is received at the current focusing position and emitted or transmitted by the target object until the central moment corresponding to the target object at all focusing positions is obtained, comparing the central moment corresponding to the target object at all the focusing positions, and determining the focusing position that corresponds to the minimum central moment as the focusing position.