Abstract: A structure of silicon micromachined Fabry Perot cavity is introduced. Through the movement of the elastic membrane as the upper mirror, the cavity length is adjusted. Under different parameters, the mechanical characteristics and the coupling electrical field of its structure are simulated and analyzed by means of the software finite element. From the results of analysis it is seen that the elastic membrane has enough strength to bear the force of electric field produced by the applied voltage in the working range. The elastic membrane can still be kept parallel with the lower mirror in the movement, parallelism of the cavity and the working precision in the device, i.e. the filter or attenuator, can be guaranteed. It offers an effective reference for the structure parameters in the design.