A mode locking device of fiber laser includes a bar structural body, having a central hollow region, extending along a reference line of the bar structural body. The central hollow region has two ends coupled with two collimators on a fiber laser loop. A polarization dependent isolator is disposed within the central hollow region of the bar structural body such that the laser ca propagate only in one direction. Several rotors are disposed within the central hollow region of the bar structural body. Each rotor has a protruding piece, for rotating the rotor along the reference line. The laser beam travels along the reference line and passes through the rotors. Several retardation waveplates are disposed on the rotator structures and can be rotated together with the rotors. Each waveplate is adjusted to an angle, such that the laser can be mode-locked automatically.