The lubrication problems near the top dead centre(TDC)of cylinder liner and piston ring in diesel engine were investigated in a piston ring reciprocating liner test rig,and the friction force sliding speed characteristics of lubricating film under different loads and lubricating medium conditions were analyzed.The results show that minor speed change results in the obvious variation of lubrication characteristic near TDC.When the load is 10 MPa,the friction force is reduced by ZDDP additives at sliding speeds less than 04 mm/s,and as the sliding speeds is more than 04 mm/s,the influence of ZDDP additives on the lubricating film shear characteristics becomes weakened.When the load is 20 MPa,the friction force is constantly reduced by ZDDP additives at the test sliding speed range.