Abstract: The constant temperature cooling water control system of HLS-Ⅱ is used to control the cooling water temperature change, and reduces the effect of the external environment on the equipment to improve the stability of the light source operation. The system is a distributed control system based on EPICS, and uses a serial port Ethernet converter MOXA6450 to connect digital indicating controller UT55A-20 to the LAN. The temperature control is implemented in UT55A-20, and IOC runs on VMWare virtual machine. The system has been stable and reliable since 2015, and the control precision of cooling water temperature reaches ±0.5 ℃ design indicators. The system provides the necessary technical support for the stable supply light of HLS-Ⅱ.