In a conventional liquid cooling garment (LCG), overcooling of the water inlet temperature shortens the working time and worsens thermal comfort. Such problems have not been well solved so far. In this study, we propose a smart cooling garment with a developed temperature regulation system, effectively reducing unnecessary loss of power consumption and hence extending the work duration. Testing on a thermal manikin was conducted to evaluate the performance of temperature-regulating LCG. The results showed that, compared to the conventional LCG, the proposed system achieved the rapid and accurate adjustment of water temperature, improved the working time by more than 37% with the total weight barely increased, and ensured the thermal comfort of the wearers. The developed LCG opens the possibility for the smart control of the temperature, fitting for the user's preferences regarding the working time and thermal comfort sensations.