<p>The mixed lubrication is the main lubrication method for gear systems. A new dynamics model for multi-stage planetary gear train considering lubrication, surface roughness, and backlash is proposed. The oil film stiffness is obtained by the elastohydrodynamic lubrication theory, the contact stiffness is obtained by Hertz theory and fractal theory. The influences of lubrication and surface roughness on total stiffness and dynamic characteristics of the system are studied, and the dynamic characteristics are verified by experiments. The results show that the larger fractal dimension, the smaller characteristic scale coefficient, and the film thickness with a range of 0.6μm to 0.8μm can increase the total stiffness, which is beneficial for reducing the vibration of system. As the rotational speed increases, the mixed lubrication enhances the chaotic characteristics of planetary gear train. The fractal dimension has a greater impact on the chaotic degree of the first stage planetary gear train than the second stage planetary gear train. The accuracy and effectiveness of the dynamic model are verified by the time-domain and frequency-domain analyses of the dynamic characteristic experiments.</p>