The modular interference beat length in polarization maintaining photonic crystal fiber(PM-PCF) is a very important parameter for designing fiber sensors based on modular interference.It plays an important role in deciding length modulation,linearity,dynamic range and modular interference sensitivity of fiber sensors.This paper calculates the mode propagation characteristics and modular interference beat length theoretically.The propagation characteristics of several low order modes and two orthogonally polarized cut-off wavelengths of LPe11 in PM-PCF are demonstrated.The relationship between wavelength and modular interference beat length of the PM-PCF is obtained.The experiment is designed to investigate the modular interference beat length in a PM-PCF,and the experiment results show that the beat length is about several hundred microns.