The invention discloses a method of measuring beat length of polarization maintaining optical fiber comprising three steps: 1) setting up test light path: broad light emitted from broad light source, via a polarizer of tail optical fiber of polarization maintaining optical fiber, through PM_PM optical melting point, enters polarization maintaining optical fiber to be tested and is connected with adegree of polarization tester via an optical fiber connector; 2) test of length of polarization maintaining optical fiber to be tested and degree of polarization data: opening the degree of polarization tester, recording degree of polarization data displayed on the tester; picking up the optical fiber to be tested from the optical fiber connector, measuring length of polarization maintaining optical fiber to be tested and recording the same; then introducing short polarization maintaining optical fiber to be tested to the optical fiber connector, then recording degree of polarization data displayed on the tester again and length of optical fiber until the length is zero; 3) function parametric fitting, acquiring beat length of polarization maintaining optical fiber to be tested. The invention uses depolarization theory of board light and less cost broad light source as well as degree of polarization tester, which is short in test time and accurate in result.