An acousto-optic Q-switched pulsed fiber lase r experimental system is set up and the influence of the duty cycle of the drivin g signal on the output pulse repetition frequency is demonstrated by adjusting the duty cycle of the driving signal of the acousto-optic moudulator (AOM).The results show that when the frequency of the driving signal is fixed,the output pulse appears at the fi rst,and then third sub-harmonics of the AOM repetition rate in order with the decrease of the duty cycle of the driving signal.The experimental results show that appear in an acousto-optic Q-switched pul sed fiber laser,the output pulse repetition frequency is influenced by the frequency and the duty cy cle of the AOM driving signal at the same time.