Abstract: Abstract: In order to investigate the effect of helical grooved structure on the clearance flow and pressure balancing characteristics of the piston - copper sleeve pair, the flow field numerical simulation of pair was carried out by using the computational fluid dynamics method. Combined with the actual structure of a specific type of swashplate pump, 6 kinds of pistons were designed with different surface structures distinguished from length and location of helical groove. At different rotate velocity, oil supply pressure, piston location, oil film pressure distribution, tilting torque size and clearance flow characteristics of 6 kinds of piston-copper sleeve pairs were analyzed and compared. The results showed that the oil film pressure distribution of helical grooved piston - copper sleeve pairs became more uniform and stable due to the effect of helical groove, and the maximum value and rangeability of tilting torque decreased observably. Specifically, when the rotating speed was 1000 r/min and the plunger position was 90°, relative to the non-grooved plunger, their change amplitude of circumferential pressure decreased by 24.05%-55.77% at axial position of 25 mm, and the maximum tilting torque reduced by 49.01%-103.14%. In all groove design proposals, compared to the single-turn circle groove, oil film pressure distribution of single-ring helical groove was more conducive to piston's stable work. Then, pressure balancing characteristic of the single-ring helical groove would be more effective as the distance between the starting point of groove and the end face of the piston increased. Furthermore, compared to the single-ring helical groove, multi-ring helical groove possessed superior balance pressure characteristics. Especially, when multi-ring helical groove was arranged continuously, the uniformity and stability of oil film would be enhanced further. In addition, the pressure balancing effect of the helical grooved structure could reduce the flow resistance of the oil in the clearance, and was good for the oil to fill the whole clearance, so the characteristics of pressure balance and lubrication would be promoted. But the sealability of piston - copper sleeve pair would be worse and the leakage would be increased by 22.73%-267.53% in the condition of high pressure and low speed. The design scheme of the three-ring helical groove obviously improved the pressure lubrication characteristics, but it seriously affected the sealing of the piston pair, which did not satisfy the working requirement of the piston pump. The size of the pressure flow in clearance increased with the depth of the groove, and the increasing speed gradually elevated before the depth reached 0.9 mm, and then the linear change rule appeared after it. Meanwhile, the flow characteristics of the helical groove were also changed with the depth of the groove. Therefore, by selecting groove size reasonably, lubrication and leakage could be controlled, which was helpful to improve piston work quality. Moreover, a method combining dynamic and static conditions was proposed in this study for proving the rationality of the numerical simulation. On the one hand, a theoretical formula was used to calculate the flow rate of concentric annular clearance at different piston velocities, and the result was compared with the flow rate obtained by the dynamic numerical simulation. On the other hand, the measurement test of clearance flow of the helical grooved piston in the static state was carried out to verify the rationality of specific helical grooved piston's numerical simulation. Probative results showed that simulation method accorded with the need of research rationality. Firstly, the calculation results of formula were in good agreement with the simulation. Then, the value difference of simulation and test was relatively large, but the change trends in comparison were consistent. The study was aimed at providing a reference for the helical grooved piston design that synthetically considered the tightness and pressure uniformity. And the groove type piston was expected to obtain good lubrication characteristics while the leakage dose did not affect the piston pump work efficiency. To promote the practical application of the helical groove type piston, the relevant research of dynamic test method and platform needs to be further performed.