In micro optical electromechanical systems, the air thickness of a Fabry-Perot cavity is about one wavelength. Therefore, the interference order is less and the reflection phases of the reflectors have much influence on the peak wavelength. The linear relation between the thickness of an air cavity in a tunable filter and the resonant wavelength is deduced and verified by numerical computation. The factors influencing the wavelength modulation sensitivity are analyzed. The relation between the refractive index, number of layers of the reflectors and the linear coefficient is derived.