The intensity of a fixed point in the interference fringes is closely related to the phase change of interference fringes. According to this principle, a real-time phase locking system was established by the analysis of the intensity field. The intensity voltage of a fixed point in the interference fringes was detected as a feedback signal by a photodetector, and an acoustic optical modulator was used to shift the frenquency of the Gaussian beam in real time. By setting intensity voltage to a fixed value, phase locking function was realized. Theoretical model of fringe phase locking system was derivated and verified by experiment. A phase locking controller was also designed based on the model. The experiment results illustrate that, the phase drift is within ±0.04 period under the 400 Hz control frequency, which satisfies the exposure requirements of interference lithography.