To reduce the pressure mutation in transition zone of high-pressure gear pump, a parametric design method for floating lateral plate damping structure was proposed by studying the relation between the transition zone pressure variation and the damping structure of floating lateral plate. Taking the model of high-pressure external gear pump CBAk as an example, the transition zone flow field of it was analyzed. With the method of response surface methodology (RSM), an approximated pressure variation model of the transition zone of CBAk was established. With the guarantee of volume efficiency of gear pump, an optimal design to the damping structure of high-pressure tank on the floating lateral plates was put forward with an optimization goal focusing on the minimum pressure variation. Results showed that the pressure mutation of transition zone was reduced from 10.162 MPa to 3.670 MPa.