The plunger is one of the most important precision coupling in the high pressure common rail pump, oil film characteristics between the plunger and the plunger sleeve directly affects the service life of the pump. The radial force on the plunger can make a bad influence on the thickness of the oil film which may cause bad lubrication and wear. The study on the thickness of the oil film is helpful to explain the cause of wear failure. Through the stress analysis of the plunger and the analysis of the oil film pressure, the change of the oil film thickness was obtained. The oil film thickness distribution in different plunger positions was calculated by simulation, and the parts which were easy to be worn were obtained, and the influence of different operating pressure on the oil film thickness was studied. The worn piston pairs were scanned by electron microscope, and their wear mechanism was analyzed. The results show that when the cam is turned 180°, the upper part of the plunger is thinner, where it is most easily worn; when the cam is at 0°,the oil film at the lower end of the plunger sleeve is thinnest; with the increase of working pressure, the film thickness decreases. Scanning electron microscopy shows that the main wear mechanism is adhesive wear and ploughing effect of hard particle.