Fiber optical Fabry-Perot (F-P) interferometric sensors have been used for the measurement of various physical parameters. A novel miniature asymmetric F-P interferometric cavity applied to fiber optical sensors is designed in this work; and its structure consists of fiber, complex metallic thin films, air or dielectric tunable layer, in which the reflectance of the cavity in response to phase thickness of the tunable layer can be approximated to saw wave function so it can improve the degree of linearity and the dynamic range of the sensors. The measurand to sense, which modulates the optical path of the tunable layer to change the reflectance of the structure, can be detected and demodulated. The response of the reflectance of the structure to the phase thickness of the tunable layer has been calculated and analyzed with optical thin film characteristic matrix method, and an expression for the response of this structure has been described. The design method to obtain optimal parameters of the F-P interferometric cavity has been concluded in this work. The result indicates that this miniature structure possess a high degree of linearity, sensitivity and dynamic range.