An undamped mode of pre-stressed straight beam was used as spectral function of Galerkin mapping to solve the dynamic problem for electrostatic-nonlinear beam coupling. The dynamic process of Pull-in and Pull-off for device was simulated. The proposed model was validated by comparing with the finite element results and the literature results. It was indicated that the proposed model not only had fast simulation ability with different action loads, but also suit for fast predicting the device characters variation with the different geometry size and different material parameters. The calculating results had sufficient accuracy and the computational costs were significantly reduced.