Abstract: To address the significant measurement errors caused by optical axis misalignment due to eccentric lens assembly in the optical imaging system of large step-shaft diameter measurement equipment, a symmetric dual-telecentric optical path dual-CCD measurement method with error compensation using a plane mirror is proposed. The principle is as follows: a symmetric dual-telecentric optical path dual-CCD optical imaging system is designed with a measurement range for diameters between 600 mm and 800 mm. A plane mirror is introduced between the collimating beam expansion system and the optical imaging system to reduce angular errors in the optical path, thereby correcting the coaxiality error of the system's optical axes, improving beam collimation, and enhancing the system's imaging quality and measurement accuracy. According to the design results, when the Nyquist frequency of the optical imaging system is 71.4 lp/mm, the modulation transfer function (MTF) value exceeds 0.65, the field curvature is less than or equal to 0.2 mm, and the distortion is less than 0.13%. After the designed optical system is integrated into a large step-shaft diameter measurement device, actual measurement results demonstrate that the improved optical imaging system can achieve approximately double the measurement accuracy compared to the original equipment.