Abstract: A β-ray monitoring system was developed, and the investigation was carried out for continuous on-line monitoring of resin content in carbon fiber/phenolic resin prepreg cloth. The calibration relationship between the values of resin content and the output signals of the β-ray monitoring system was proposed. Moreover, the effects of the steadiness of the β-ray generator and the deviation of carbon fiber cloth on the measurement accuracy were theoretically analyzed. The results show that highly accurate on-line monitoring of resin content can be performed in the continuous manufacture process of the carbon fiber/phenolic prepreg cloth.