The dynamical behavior of MEMS devices is strongly affected by viscous fluid damping effects from the surrounding. These damping effects have to be carefully considered for the design and optimization. For predicting the hydrodynamic loading of an electrostatical device in viscous fluids, it is necessary to build an electrostatic-mechanical-fluidic coupled model, where the flows characteristics were described by Navier-Stokes equations. Comprehensive analyses of the static/dynamic closing behaviors of the microswitch in viscous fluids were performed. Compared with published work of other researchers available in the literature, the numerical model was validated. After that, the static/dynamic closing voltages of the microswitch in the air and deionized water were calculated. The transient behaviors of microswitch under different action voltages were also simulated, including the squeeze-film damping pressure distribution as well. The results show that the difference between the static and the dynamic pull-in voltage in the deionized water is smaller than that in air.