The operational principle for resonant cavity of fiber optic Farby-Perot is analyzed, in addition to the selection for its operating point. The relationship of optical phase sensitivity of resonant cavity and its fineness was deduced. The theoretical formula and curve are presented. The above system is compared with the interference system of fiber optic Math-Zehnder,the great difference of the two interference system is pointed out as phase sensitivity considered.The unique advantages of fiber optic Fabry-Perot interference system are demonstrated. The transmittance curve of fiber optic Fabry-Perot interference and fiber optic Math-Zehnder interference were calculated and plotted with Mathcad software.The experimental system of fiber optic Fabry-Perot interference as well as the transmittance curve pictures taken in experiment is introduced. A method was given,with which the effective fineness calculation can be achieved. The experimental pictures was well in accordance with theoretical cures.