Abstract: In solving problems of seal failure in composite transmissions,based on the flow field analysis of rotary seal and force analysis on the sealing ring in working condition,state of the motion of the sealing ring under the condition of boundary friction is analyzed.The relational expression for judging the state of motion of the sealing ring is deduced.Analytical results indicate that the motion state depends mainly on the structural parameters of the rotary seal,friction coefficient of friction pairs,and operating oil pressure of the wet transmission clutch.State of motion of the sealing ring in working condition is measured dynamically by eddy current displacement sensor.Experiments demonstrated the validity of the theoretical conclusions deduced.It provides theoretical basis for the design of rotary seals.