Dynamic characteristics of typical actively Q-switched fiber lasers are numerically investigated by using the traveling-wave method. The multipeak phenomena of output pulses are analyzed, and the mechanism for these phenomena is detailed. In particular, the switching-induced perturbation and its influence are emphasized under different conditions. A quantitative analysis shows that under certain conditions of cavity, pump and switching, a split-pulse output can turn to a single-pulse output. Then how to realize the single-pulse output is systematically illustrated, and some experimental results are shown for comparison.