Abstract: A study was conducted to address the performance degradation of multi-stage step seals in hydraulic machines, which is caused by back pressure accumulation during reciprocating motion. A two-dimensional axisymmetric finite element model was developed to obtain the macroscopic contact pressure distribution, and a mixed elastohydrodynamic lubrication (mixed EHL) model was established to investigate the evolution of sealing behavior under different back pressure levels. The effects of back pressure on the contact pressure, leakage, and friction at the sealing interface were analyzed through numerical simulations. The results indicate that the contact length of the primary seal is reduced and the stress concentration at the lip section is intensified as the back pressure increases, leading to interfacial separation failure at a critical back pressure of 5 MPa. Meanwhile, the frictional load on the secondary seal is increased due to its enhanced bearing capacity, triggering a functional shift from the primary to the secondary seal. Theoretical insights and practical guidance are provided in this study for enhancing the reliability and structural optimization of sealing systems in hydraulic equipment.