Abstract: The beam mobile five-axis gantry milling machine tool is a kind of high-speed milling machine tool with double-swing head five-axis, which is suitable for large and heavy workpieces, as the end of the machine tool transmission chain, its swing axis is directly connected with the cutting tool. Therefore, the position-dependent geometric error detection and identification of swing axis is of great significance to improve the machining accuracy of the large five-axis machine tool. Took the XKAS2525×60 five-axis gantry milling machine tool as the research object, established the kinematic model of the machine tool swing axis (B axis) by using the multi-body system theory, and the geometric error identification model of machine tool swing axis is established based on the double ballbar. Finally, through the experiment, six position-dependent geometric errors of B-axis are detected and identified, and the experimental results were analyzed by regression and comparative. The results show that the proposed method for the geometric error detection and identification of machine tool swing axis is reliable and effective, the results of detection and identification are correct.