The characteristics of a Q-switched ytterbium-doped double-clad (YDDC) fiber laser are presented. Due to the self-mode-locking effect in the Q-switching process, the Q-switched pulses split up. Their initiation and evolution are experimentally analyzed, and the mechanism is detailed. Based on the proposed mechanism, an effective model is developed to simulate this Q-switched YDDC fiber laser and to reconstruct simultaneously Q-switched and mode-locked pulses. The simulated results agree well with the experimental results.