In order to improve the lubrication performance of the cylinder/valve-plate interface of a small axial piston pump, an oil film lubrication model was established based on the force analysis of the cylinder block. With leakage, cylinder overturning angle and viscous friction torque as the optimization objectives, the structural parameters of the valveplate seal ring were optimized by multi-objective non-dominated sorting genetic algorithm (NSGA-Ⅱ)， including the radial dimensions of valve-plate sealring R1,R2,R3,R4 and the starting angle θ of the waist groove.Considering the micro tilt motion and macro rotation motion of the cylinder block, the dynamic change process of the wedge oil film was simulated.The pressure distribution of the oil film on the cylinder/valve-plate interface was obtained by discretizing the Reynolds equation by the finite volume method. The results showed that the size of the outermost edge of the seal ring had little influence on the lubrication performance of the oil film, while the starting angle of the waist groove and the size of the inner edge of the seal ring had large influence on the overturning angle of the cylinder. After the optimization of the structure of valve plate, the comprehensive lubrication performance of the oil film was improved by 5.4%, and the overturning angle and leakage rate were decreased by 3.8% and 29.6%, respectively.