In order to improve the grinding accuracy of a CNC camshaft grinder and ensure that the camshaft profile error meets the required standard, the motion error and thermal error of the grinder were analyzed and studied. The motion relation between the main moving parts of the camshaft grinder was analyzed, and a method of coupling the motion error and thermal error of the camshaft grinder was put forward. This method is universal to the grinder, and the source of the error can be directly observed, which is very effective for error compensation. Firstly, the multi-body system of grinding machine was abstracted by using multi-body theory and coordinate change method. Secondly, the main grinding parts of grinding machine are divided into two moving chains, namely the “work-bed” chain and the “grinding wheel-bed” chain. The motion chain and the grinding point were combined into a closed-loop system, which was a simplified grinding motion process. Finally, the coordinate system of key moving parts was established, the transformation matrix of moving parts was obtained, the thermal error and geometric error were coupled, the constraint equation of precision machining was established, and the comprehensive error model of camshaft grinding machine was derived, providing the theoretical basis for machining compensation. The method has been verified in practical production, and the results show that the grinding accuracy of the compensated grinding machine is increased, and the error of the actual camshaft profile is significantly reduced after testing.