The geometric parameters don’t reflect the physical significance enough although there are a lot of different geometric parameters in the study of mechanical seal with microdimples.Therefore,the common geometric parameters which are independent of the dimple shapes were investigated.Based on the multiphase flow cavitation model,threedimensional numerical models of the lubricant in mechanical seals with rectangle,diamond and elliptical dimples were developed,and 144 data of opening force were obtained.Based on the analysis of the relationship between the opening force and the geometric size,and the mechanism analysis of additional opening force,two geometric parameters with physical significance,namely equivalent length and equivalent width,were presented.The grey correlation analysis was employed to analyze and verify the influence of equivalent length,equivalent width,orientation angle and area ratio on the opening force.It is found that the four factors have impact on the opening force of three microdimple shapes.Moreover,the equivalent length and equivalent width have greater impact than the other two.