To enhance the stability of the fiber grating refractive index sensor and increase the grating reflectivity, the Polarization Maintaining Microstructured Optical Fiber (PM-MOF) grating Fabry-Perot (F-P) cavity is proposed. Based on Finite Element Method (FEM) and Transfer Matrix Method (TMM), the refractive index sensing characteristics of the sensor is analyzed. The relationship between the refractive index and resonant wavelength difference is simulated. Moreover, the dependence of the reflection spectrum on the center big holes size and the F-P cavity length is studied. Since two polarization modes respond similarly to the outside perturbation, the fiber possesses high stability. Based on the interference, the reflectivity is enhanced obviously, and almost remains unchanged as the analyte refractive index increasing. It is beneficial for the detection. The results provide the theoretical basis for the application of PM-MOF grating in the optical fiber refractive index sensor and the optical fiber label-free biosensing.