The measuring system for chlorine utilization was built on a minitype jet singlet oxygen generator （JSOG）. The influence of light-source bandwidth, gas temperature, water vapor content and other gas components on the chlorine utilization measured by UV absorptiometry was experimentally and theoretically studied in detail. For a well designed experimental setup, the relative error for the chlorine utilization measurement was about ±2%, which mainlv caused by the noise of the photomultiplier （PMT）. On the basis of error analysis, a method to further decrease the measuring error and also extend the measuring range of chlorine utilization was proposed. This study will be helpful for the accurate chlorine utiliza- tion measurement in the singlet oxygen generator （SOG） for a chemical oxygen iodine laser （COIL） system.