Abstract: Ultra-precise rotary axes are extensively utilized in advanced equipment, and their motion errors can significantly impact the accuracy of these devices. Traditional measurement techniques for rotary axes errors typically involve high-precision standard devices as references, but these can inadvertently reduce measurement accuracy due to inherent shape errors. Furthermore, error separation techniques are both cumbersome and time-consuming. Optical-based measurement methods, though useful, often fail to achieve high accuracy, especially in measuring axial motion errors. This paper addresses the impact of rotary axes motion errors in ultra-high precision aspheric measuring instruments and reviews two existing measurement methods. It introduces a novel measurement approach based on a composite laser target capable of assessing five degrees of freedom in motion errors, including axial, radial, and angular errors. This method employs a composite laser target equipped with a laser point light source and a collimated laser beam, affixed to the rotary axes as a reference datum. The method's efficacy is demonstrated in accurately determining the axes' position and orientation by measuring the target's position and angle. The differential confocal microscopy technique is applied to ascertain the axial position of the laser point source, thereby determining the axes' axial error, while a traditional microscope optical path measures the radial position for radial error. Collimation measurement optical path is used to evaluate the laser collimation beam's angle for angular error assessment. The method exhibits resolutions of 4 nm for axial, 2 nm for radial, and 0.2 μrad for angular errors. Moreover, the feasibility of this method in measuring rotary axes motion errors is validated through tests on an air spindle. Overall, this approach replaces traditional standard devices with an optical reference device, eliminating the need for additional error separation processes and facilitating real-time monitoring of rotary axes motion errors in ultra-high-precision measuring equipment.