Abstract: The precision machining of large-scale critical components for aerospace high-end equipment heavily relies on large gantry machine tools. Thermal errors in fully closed-loop feed axes directly compromise the thermal stability of these machines, often leading to inconsistent machining accuracy. A gantry-type five-axis machine tool from an aerospace enterprise is investigated, focusing on thermal error characterization, modeling, and compensation of its fully closed-loop feed axis. Firstly, the structural configuration and operating principle of the grating scale are analyzed, and the thermal error of the fully closed-loop feed axis is decomposed into thermal expansion error and origin thermal drift error. Secondly, thermal errors and temperature variations are measured and analyzed under both motion-induced heating and static cooling conditions, leading to the development of separate models for thermal expansion error and origin thermal drift error. Finally, the coordinate origin offset method is adopted to conduct thermal error compensation experiments with the cooling system on and off for verification. The results demonstrate that the proposed compensation method achieves high prediction accuracy and robustness, significantly enhancing the thermal stability of the machine tool and providing an effective solution for improving machining precision consistency in large gantry machine tools.