In order to achieve high-precision and fast measurement of parallelism of flat glass， a new measurement method using an area growth algorithm to segment the interference fringe map was proposed. A Fischer interferometer was used to collect the self-interference fringes generated by reflections from the front and back surfaces of the flat glass， and an area growth algorithm was used to segment the collected self-interference fringes， extract the self-interference fringes， and statistically count the number of interfering fringes within the measurement aperture， so as to realize the measurement of the degree of parallelism. After comparing the region growing algorithm with the Sobel algorithm， it was found that the region growing algorithm performed better in the extraction of interference fringes. Measurements of different types of interference fringes using this algorithm and comparisons of the measurement results with those of ZYGO showed that the error of parallelism measurement based on the region growing algorithm was less than 1″.Using this interferometric method of image processing， the parallelism of flat glass can be measured easily and quickly， which improves the inspection efficiency of flat glass， and at the same time has a high measurement accuracy.