As for an interference optic fiber sensor, the characteristics of its performance are directly influenced by the spectrum width of light source. Based on the research of Fabry-Perot interference optic fiber sensor, its sensitivity is theoretically formulated. Futhermore, with the application of the mathematical analysis of the result by software MathCAD, the problem of how sensor sensitivity being influenced by the spectrum width of light souce is discussed. An experimental system of optic fiber interference with feedback function is introduced and is applied to the display of a resonance curve. Two important experiments with the application of the above system (one is a comparison experiment on the sensitivity of interference cavities of different lengths; the other is a light source experiment on different interference lengths) present