In order to minimize the thermal elongation of high-speed spindle of gear form grinding machine, an optimization method for thermal behavior is proposed in this paper. The influence of constrained modes of bearings on the positional accuracy of the spindle is discussed, and the optimum constrained mode is obtained. An objective function of thermal contact resistance is established according to the semi-empirical formula to optimize the routes of heat transfer in the spindle assemblies through the constrained optimization method. The simulation and experimental results show that the temperature rise of the grinding wheel shaft and its associated components are all very small, and the thermal elongation of the grinding wheel shaft is about zero.