Laser-diode pumped Q-switched ytterbium-doped fiber laser is studied experimentally by controlling the switching time of acousto-optic modulator (AOM). The characteristics of Q-switched pulses with different rise time of AOM regulated by the laser beam size along the window of AOM are presented. Meanwhile, the behaviors of Q-switched pulses are achieved by regulating the switching time of AOM. The single-repetition-rate and half-repetition-rate phenomena are described and discussed. The experimental results confirm that the fiber laser with lower level inversion population can be more easily operated for half-repetition-rate generation.