The theoretical model is established for photonic crystal fiber (PCF) intrinsic Fabry-Pérot (F-P) interferometric fiber pressure sensor, and its pressure responsibility, related to several parameters, is also discussed with the results simulated by Matlab software. The improvement of single/multi-mode intrinsic F-P interferometric fiber pressure sensor is demonstrated by theoretical analysis, which is easier to fabricate. The structure is constructed by fusing single-mode fiber and multi-mode fiber with different diameters. The theoretical model of pressure response is also given. The key parameters concerned with pressure responsibility are analyzed with simulation curves. By the comparison between two kinds of sensors, the single/multi-mode intrinsic F-P interferometric fiber sensor is more attractive because of its high responsibility and simple fabrication.