Abstract: In order to study the symmetry property of a single pulse for passively Q-switched lasers,the coupled rate equations were solved numerically and the temporal profile of laser pulse was obtained.The relationship between the symmetry property of a single pulse and the parameters of absorber and output coupler was presented and the pulsewidth was recalculated accounting for the asymmetry property of laser pulse.The relationship between pulsewidth and parameteres of absorber and output coupler was presented also.Conclusion was drawn that the pulse was symmetric when the variable b=0.6 and that the pulsewidth was minimal when b=0.78.This provides a method to optimize the pulse shape and pulsewidth by adjusting the value of b.