The principle of polarization maintaining fiber (PMF) is analyzed and a three-dimensional model is established by utilizing the finite element method (FEM). The stress distribution and normalized birefringence are obtained through this model. Three pure-optical beat-length measuring systems are carried out for contrastive research and the advantages and disadvantages are described. Experimental results are in good agreement with the theoretical results. Then one simple and effective method to test the beat-length of high-birefringence photonic crystal fiber (Hi-Bi PCF) is chosen and the results indicate that its beat-length is about 1.2 mm. This research provides an effective theoretical method for the analysis of PMF and a convenient experimental system for the beat-length testing of complex structure PMF and Hi-Bi PCF.