Epoxy thin films have been widely used in microelectromechanical systems, aerospace and civil engineering, which are exposed to external excitations in wet environment that lead to severe nonlinear vibration. In this paper, a multiscale simulation consisting of molecular simulation and meshless simulation is adopted to study moisture effect on nonlinear vibrations of epoxy thin film. In molecular simulations, the cross-linked epoxy molecules with moisture content from 0.0 to 4.0 wt% are constructed. It is measured that mechanical properties of epoxy molecules show an initial enhancement with moisture content from 0.0 to 1.0 wt%, and a subsequent decrease when moisture content increases to 4.0 wt%. With molecular simulation results as inputs, meshless simulations are carried out to investigate epoxy vibration behaviors, where vibration equations of epoxy thin films with investigated moisture contents are constructed and solved. It is determined that fundamental frequencies of epoxy thin films show a similar trend as the variation of mechanical properties. Meanwhile, the level of nonlinear frequency ratio decreases in 1.0 wt% case and subsequently increases up to 4.0 wt%. The vibration behaviors of epoxy thin film as revealed in this work contribute to the prediction of vibration behaviors of epoxy-based applications in wet environment.