Abstract: The isotope separator on-line (ISOL) equipment is a part of HI-13 tandem accelerator upgrade project, and it mainly produces radioactive nuclear beam, identifies radioactive nuclide and injects radioactive nuclear beam into 13 MeV tandem accelerator. It has high mass resolution. In order to realize the monitoring task of the equipment, a distributed control system based on Ethernet was designed under the framework of PLC according to the distribution and characteristics of ISOL. The characteristics of device, structure of monitoring system, control mode of device, design of safety interlock and anti-interference, graphical monitoring software design, data storage and running situation were introduced in this paper.