Abstract: Regarding the technical challenge that the temperature distribution law and cooling effect of the high-speed motorised spindle in horizontal machining centres under reciprocating axial water channel cooling remain unclear, a fluid-structure interaction (FSI) simulation model was established to systematically investigate the steady-state thermal distribution characteristics and corresponding thermal deformation laws of the motorised spindle at multiple sets of commonly used rotational speeds. The simulation results are then compared with experimental data for verification.The research findings indicate that when the rotational speed is 2 000 r/min, the overall temperature rise of the motorised spindle is relatively small, and its thermal performance tends to stabilise. When the rotational speed increases to 16 000 r/min, the maximum temperature rises to 58.369 °C, with the temperature rise rate accelerating significantly as the rotational speed increases. From the perspective of local thermal distribution, the temperature of the bearings near the motor area remains at a relatively high level, with the maximum temperature difference between the two front bearings reaching 0.896 °C. In terms of thermal deformation characteristics, the thermal deformation magnitude of the motorised spindle increases with the rotational speed, and the maximum thermal deformation reaches 67.103 μm. The simulation analysis results and experimental verification conclusions obtained in this paper can provide reliable theoretical basis and data support for the subsequent optimisation of motorised spindle thermal deformation control strategies, the design of thermal compensation schemes to improve the accuracy stability of horizontal machining centres, and the improvement of cooling systems.