Q-switched rare-earth-doped single-mode fiber lasers attract much attention in many applications. It is well know that in actively Q-switched solid-stage lasers, single output pulses observed in experiments are in good agreement with analytical solutions and numerical simulations. However, in actively Q-switched fiber lasers, the output pulses break into several sub-pulses in Q-switched envelopes. These phenomena have been reported in various actively Q-switched fiber lasers under different cavity schemes, pump and fiber conditions. Since dynamic distributions and evolutions of photon density and inversions in the laser cavities have not been effectively measured at the initial stage of Q-switching, the mechanism of these phenomena has remained uncertain in past two decades. Since the understanding of the related mechanism is critical to the design and development of laser systems, some conjectures have been proposed in the literature, based on experimental observations and analyses. Unfortunately, without careful examinations on the pulse initiation and evolution processes, the mechanism has not been exactly explained. In this paper, by investigating the initiation and formation of split pulses in typical actively Q-switched fiber lasers, the mechanism is exactly illustrated for the first time to the best of our knowledge. It is related to the evolution processes of the injected perturbation, and determined by the rise time of switching and conditions of photon density and inversions in Q-switching processes. Furthermore, based on this principle, some solutions to realize single-pulse output are proposed in this paper, and the simulated results are compared with the experimental ones.