To improve the modular interference sensitivity and temperature stability, a Polarization Maintaining Photonic Crystal Fiber (PM-PCF) was used to design the modular interference sensors. The modular interference in a PM-PCF was investigated, and the relationship between phase difference of LP01 and LP11 even mode and interference output intensity distribution was given. The relationship plot of relative effective refractive index and wavelength was got. An experiment was set up to demonstrate the intensity distribution of far field interference output from the PM-PCF. The experiment results show that the modular interference output lodes’ intensity of PM-PCF will exchange with each other when strain is applied on the PM-PCF. This phenomenon can be adapted for designing novel optic-fiber strain sensors.