When a four-quadrant detector is used to receive interference fringes to measure the displacement,the shapes of the interference fringes will affect the phase difference between the detectors.Based on the relationship model between the interference fringe shape and the interference angle, a method to recognize the shape of an interference fringe by the four-quadrant detector was presented. The theoretical formula about the shape parameters of interference fringes with the phase difference of signals obtained by the detector was deduced using the area integration of interference fringe intensity, and the relationship between phase difference and interference angle was given.The interference fringes were dynamically modulated by using a PZT to drive the reference mirror uniformly,and the recognition accuracy of phase difference of the signal was improved.As the phase difference from 0toπ was recognized in high precision through the ellipse fitting technology,and the phase difference in the range(-π,π)was recognized by combining a specific sinusoid and a cosine integral calculation,the deflection angles were measured successfully.Compared with the traditional recognition method thatuses the shape of CCD stripe,this method expands the measurement range of the angle,so it is more suitable for dynamic measurement.Experiment results demonstrate that the precision of the proposed method is 3″for the range from-300″to 300″,and the method owns higher precision as comparing with a high-precision autocollimator.