To develop a highly efficient and energy-saving cooling system in a data center, this study investigates the optimal thermal management both on the internal server cooling cycle and external cold source water cycle under different ambient temperatures based on the principle of minimal power consumption. Taking a server cabinet of 4.8 kW as the case, an internal indirect water-cooled heat sink cycle and an external cooling tower cold source cycle are constructed. A hybrid thermal management model combined with a heat dissipation model and a power consumption model was developed with the aid of MATLAB and TRNSYS. Using this model, the relationship between the chip dissipation performance attached to the fin-type heat sink and the energy consumption performance of the cooling system under different water working conditions and ambient temperatures was analyzed. Finally, to achieve minimal system energy consumption, the optimized inlet temperature and water flow rates corresponding to different ambient temperatures were obtained by fitting expressions that can better serve the optimal design of data center cooling systems. Furthermore, when the optimal value was applied using the year-round temperature of Tianjin, China in the case study, a 21.3% reduction in power consumption and a 4.43% decrease in the power usage effectiveness can be achieved compared to the case without optimization. Therefore, significant power can be saved in a data center cooling system through thermal management optimization.