Abstract When the high-pressure common rail radial piston pump works, the piston is subjected to film pressure and cam driving force, resulting in some micro-motion such as offset and inclination. A large number of theoretical studies have shown that the micro-motion of piston has an important effect on the film characteristics of piston-cylinder interface. In this paper, according to the theory of hydrodynamic lubrication, Reynolds equation and film thickness equation of piston micro-motion are established, and solve the problem of the lubricant film of the piston-cylinder interface under different working conditions. The results show that the working pressure and cam speed have great influence on the film characteristics of piston-cylinder interface with micro-motion. The research results provide theoretical support for the optimization of the film parameters of the piston-cylinder interface.