In order to study the effect of overlay and laser power density on shockwave in laser shock processing to AZ31B magnesium alloy,the laser induced shockwave was measured by a polyvinylidene fluoride (PVDF) gauge with short rise time and high response frequency and a digital oscillograph. Based on the piezoelectric waves measured in the overlay geometry and direct ablation regime,pressure waves were obtained and compared;the pressure waves acquired in different laser power density conditions were also compared and analyzed. The research results show that the peak pressure of laser shockwave in the overlay geometry is about as eight times as it is in the direct ablation regime,and the duration time of the pulse is extended obviously. Below the dielectric breakdown threshold of the overlay,the peak pressure of laser shockwave is improved with the increase of the laser power density.