The interference fringe period is an important phase-locking parameter in the scanning beam interference lithography (SBIL) system. To measure the interference fringe period accurately, a heterodyne period measurement method is proposed. Compared with traditional methods, the requirements for the stage motion characteristics are greatly reduced. In this paper, the theoretical error of the period measurement method is analyzed and relevant experiments are performed. The results show that the average period measurement value is 555.539 nm and the standard deviation of measurement repeatability is 2.5 pm. This method is significant for holographic grating fabrication using the SBIL system.