A device of silicon micromachined Fabry-Perot cavity is introduced. It can be extensive used in the optical fiber communication,such as optical filter and optics attenuator. The device is fabricated by general surface processes,such as LPCVD,RIE,chemical etching and evaporating metal-coating. The silicon nitride film which is suspended above the silicon substrate,is driven by an electrostatic force. The basic principle,operating range and threshold voltage of the device are analyzed,and the factors of influence threshold voltage are discussed. By means of the finite element method,the mechanical,the coupling and the response characteristics of the device are analyzed.