In order to analyze the influence of the radial micro-motion of piston-cylinder interface on the oil charac- teristic,based on the theory of dynamic pressure supporting,the oil shape in the piston-cylinder interface with dif- ferent pressure and rotating speeds as well as its change is simulated.Then,a hydraulic pump is tested by using a service life test bench,and the test results are compared with the theoretical ones.The results show that (1) with the radial micro-motion,there exists a minimum oil thickness,and the pressure in the interface is higher than that in the cylinder; (2) the thickness in the pressing area linearly increases with the increase of pressure; (3) as the rotating speed increases,the thickness in the pressing area decreases and the decrease tends to become smaller; (4) when the turning angle is 90°,the thickness reach the minimum value; (5) in the absorbing area,the mini- mum oil thickness is irrelevant to the rotating speed,and it is smaller than that in the pressing area; and (6) the position of the minimum oil thickness is consistent with the direction of the centrifugal force.