With the development of motorized spindle, the cooling effect and the distribution of cooling capacity become the crucial problem of cooling system. An optimization method for ThermoElectric-based Cooling System (TECS) is proposed based on the conservation of energy to distribute the cooling capacity. The main strategy of the proposed optimization method is to make the cold and heat input at different regions of the spindle sleeve equal in real-time through optimizing the contact area between the Heat Conduction Sleeve (HCS) and spindle sleeve. The numerical simulation and thermal characteristics experiments are carried to verify the effect of the proposed optimization method and the TECS. The simulation and experimental results show that the maximum temperature rise and thermal elongation of the TECS-based motorized spindle are reduced 56.7% and 58.6% compared with water-cooled motorized spindle, and the temperature distribution of the spindle sleeve is more uniform. It is meaningful to improve the accuracy of motorized spindle.