<p>The lubrication characteristics of planetary gear transmission systems significantly influence their dynamic behavior. Based on the Newtonian fluid assumption, an isothermal elastohydrodynamic lubrication (EHL) model was established for the contact zone of the planetary gears. The multigrid method was employed to solve for the oil film pressure, oil film thickness, and oil film deformation of the gear pair. Based on the obtained oil film parameters, the oil film stiffness was calculated and subsequently coupled with the time-varying mesh stiffness. The variation of friction factors during the meshing process of the planetary gear system under EHL conditions was then analyzed. On this basis, a dynamic model of the planetary gear transmission system was developed, comprehensively considering the effects of EHL, time-varying friction forces, tooth flank clearance, and time-varying mesh stiffness. Using methods such as bifurcation diagrams, Poincaré maps, three-dimensional phase diagrams, and frequency spectra, the influence of EHL on the system’s vibration characteristics was systematically investigated. Finally, experimental validation was conducted to verify the accuracy of the dynamic model. The results indicate that EHL can reduce the chaotic region of the system and transform certain unstable states into more stable motion patterns. The comparison between experimental and computational results demonstrates that the dynamic model considering EHL offers higher predictive accuracy, thereby further confirming the reliability of the EHL model established in this study for planetary gear transmission systems.</p>