Dynamics of split pulses in actively Q-switched fiber lasers are numerically investigated, and the mechanism that produces multipeak phenomena (MPP) is precisely illustrated. It is shown that the first transient pulse in a cavity is initiated by the quick switching of a Q-switching element, which actually injects part of the amplified spontaneous emission into the laser cavity in the form of a pulse. Then the multipeak structures are related to the evolution of the perturbation caused by the switching and are determined by the rise time of the switching and by conditions of photon density and population inversion.