The influences of amplified spontaneous emission (ASE) on the formation and evolution of pulses in actively Q-switched ytterbium-doped fiber laser (YDFL) are investigated experimentally. The experimental results show that the combined effect between the short switching time and the gain transient property of ytterbium-doped fiber (YDF) causes the initial ASE power fluctuation,which then forms the multi-peak structure in the output pulse after having been circulated and amplified in the cavity. Meanwhile,the high-level broadband ASE may give rise to the gain self-saturation effect in the YDF,making it difficult to form Q-switched laser pulses for the fiber laser. The output pulses may be eventually Q-switched ASE pulses,giving lower peak-power and larger pulse duration. By using fiber Bragg grating (FBG) to suppress the gain self-saturation effect,the gain supplied by the YDF can be greatly enhanced,making it easy for the laser to achieve the Q-switched laser pulse with low threshold,small duration,and high peak-power. When the YDFL with FBG is pumped with 160 mW,the Q-switched laser pulses with the peak power of 40.7 W and the duration of 30 ns are achieved.