The relation between the air gap thickness and the peak wavelength in micro-optical electromechanical system (MOEMS) optical tunable filter based on Fabry-Perot etalon is discussed. In the devices based on MOEMS, length of the air cavity is about one wavelength, so the phase shift on reflection of the dielectric reflector plays a crucial role in the low interference order. From the phase shift, the linear relation between the air gap thickness and the peak wavelength is presented and then proved by numerical computation. The refractive index of thin films and the number of stacks constructing the reflector that influence the tunable sensitivity are analyzed. At the end, error in a specified wavelength range is estimated.