A method based on the Senamont method was proposed to precisely measure the birefringence of an infrared crystal. Two specimens with the Optical Pass Differences (OPD) within 1 wavelength caused by the birefringence were compared to overcome the measured OPD limit of Senamont method and to extend the method to the infrared band. According to the new method, a set of apparatus for measuring the birefringence of infrared crystal was developed. Moreover, by applying the Jones matrix, the expressions of signal intensity with various error sources were proposed to analyze precisely the influences of the error sources including the azimuth error of a polarizer, the orientation error of the quarter wave plate, the azimuth error of the specimen and the orientation error of the analyzer, then the measuring precision of this method was evaluated. The experiment results indicate that the measured OPD error for a standard quarter wave plate is 0.00376 μm, and the relative error is 0.44%, which is within the precision and satisfies the system targets.