Abstract: Thenovelannular cylinder-piston system under an annular series arrangement can increase the engine power performance, such as the work frequency per rotation, but also has leaded to uneven wear on the annular cylinder wall, due to the ring’s rotation around the rotor axis. To analyze the friction and lubrication characteristics of the annular cylinder-ring system, firstly, the lubrication mechanism was studied using a wedge effect around the annular cylinder-ring contact area, and the rotation of ring was illustrated under the annular cylinder construction. Secondly, with a consideration on the annular cylinder wall construction and ring rotation, the dynamic oil film analytical model of the annular cylinder-ring structure was established, using the two-dimensional mean Reynolds equation to describe the relationship between of oil film thickness, pressure and the motion of ring. Then the convex micro-body contact model by Greenwood was utilized to analyze the balanced forces between finite small bodies, which are on different part of the ring. Thirdly, after a spatial and time discretization of the dynamic oil film analytical model, the finite difference numerical calculation method was applied to solve the equations, using a Successive-Over-Relaxation algorithm (SOR). Before the calculation, convergence conditions of oil film thickness and pressure had been defined, and initial value of oil film thickness and boundary conditions had been confirmed. After the calculation, influences of lubricating oil viscosity and piston ring tension on dynamic pressure oil film thickness, minimum film thickness ratio and oil film pressure were evaluated. The calculation results showed that while the rotor was rotating, different point on the ring installed to a rotor-piston had different velocity relative to the annular cylinder wall fixed on another rotor, resulting in different oil film thickness circumferentially along the ring body. The unevenness of the oil film thickness became worse during different thermodynamics stages. The minimum film thickness ratio and oil film pressure presented similar evolution characteristics. By increasing the lubricating oil viscosity 4 times, the maximum value of the minimum film thickness ratio increased accordingly from 2.5 μm to 3 μm during some period of the thermodynamic process, indicating that the hybrid-lubricating state had evolved into a whole-lubricating state for the cylinder-ring structure. While considering the maximum value of the minimum oil film thickness, it enlarged over 2 times. Reducing the ring tension value to half, the maximum value of the minimum oil film ratio changes from 3.8 μm to 2.3 μm, demonstrating a thinner oil film and a worse lubricating result. However, neither the lubricating oil viscosity nor the ring tension would play an important role on the variation of the oil film pressure, it was mainly determined by the gas pressure on both sides of the ring. Conclusion could be drawn that by increasing the lubricating oil viscosity or reducing the ring tension, the annular cylinder wall would achieve a better lubrication performance, such as improving the minimum film thickness ratio and decreasing the uneven lubricating state of the annular cylinder-ring structure. The research results revealed preliminarily the formation and evolution of uneven oil film between the annular cylinder and ring, and would contribute a theoretical basis for the subsequent calculation of friction power consumption and optimization design of wear reduction of annular cylinder.