Optical axis parallelism is critical to high-quality clear imaging and stable field switching during zooming. In this paper, we propose a fast measurement method of optical axis parallelism based on collimator and image processing technology. Our method improves the measurement accuracy by introducing skeleton thinning algorithm. The method realizes the automatic measurement of the optical axis parallelism during zooming and reduces subjective errors. The experimental results show that the measurement efficiency and accuracy have been improved, confirming the practical value of the method. © 2023 SPIE. All rights reserved.