The relationship between the two thresholds of inversion density in Q-switched laser,high Q value threshold and low Q value threshold is acquired. Through analyzing the relationship between pulse width and population inversion density and the relationship between diffraction efficiency and population inversion density,a conclusion is obtained that the pulse compression is gotten by increasing pumping rate only when diffraction efficiency is matched up to pumping rate and repetitive frequency in A-O Q-switched laser. And the matching formula between diffraction efficiency and pumping rate is acquired by the match condition. The diffraction efficiency in the formula is just the minimum one to effectively compress the pulse. The theoretical results are verified by the experiments.