Based on the investigation of engaging points' displacement of gear pump with three idle gears ,the radial components of instantaneous hydraulic pressure and the engaging force of the compound pump are analyzed .The research comes to the conclusions that the instantaneous radial forces acting on center and inner gears can be balanced so long as the center gear has triple times tooth ,and the idle gear of even tooth makes less radial hydraulic pulsation than those of odd ones. [WT5HZ]