In the experimental research and industry applications of the fiber Bragg grating-based Fabry-Perot interfirometric (FBG FPI) sensor, its sensitivity has been widespread concerned. Considering the linewidth of the light source, the power transmission coefficient is amended and the theoretical formula of the sensitivity is deduced. Then, the key influence factors on the sensor sensitivity are discussed, including the cavity length, the coherence length of the light source, and the reflectivity of the FBG. The results show that the smaller the ratio between the cavity length and the coherence length is, the higher the sensitivity will be. When the ratio is chosen, there is an optimum reflectivity. If the reflectivity of FBG equals the optimum, the sensitivity is the highest. The results of comparison experiment on the sensitivity with different cavity lengths and sensitivity experiment on different reflectivities verify the correctness of the theoretical simulation.