Quartz pendulum reed is the core device of the quartz flexible accelerometer, and the machining accuracy of its key feature size is determined by the machining accuracy of the pendulum substrate. The feature size of the substrate surface is routinely measured with a tool microscope. This measurement process is complex, with poor repeatability and low efficiency. In this article, a method for measuring the feature size of the pendulum substrate based on the regional gray gradient difference is proposed. Firstly, the hardware structure of the visual measurement system is designed and developed. Secondly, the edge detection algorithm of the pendulum substrate based on the regional grayscale gradient is established. Thirdly, the search and classification method of the feature edge points is proposed around the structural characteristics of the pendulum substrate. Finally, the feature size measurement experiment of the pendulum substrate is carried out. Experimental results show that the proposed edge detection method based on regional gray gradient can replace the Zernike moment method to improve the edge detection efficiency when the fitting deviation of the center of the pendulum substrate is less than 3 μm. The measurement system can complete the measurement of the characteristic size of the pendulum substrate within 2 s, and the repeatability of the measurement of the characteristic size of the pendulum substrate in different postures is better than 8 μm. Compared with the tool microscope, the reliability and repeatability of the measurement results are improved, and the measurement efficiency is significantly improved.