The CRF circulating pump gearbox system of nuclear power plant was taken as the object,and the monitoring results and reliability of the oil online monitoring system was analyzed.Based on the working conditions and characteristics,monitoring indicators was selected,and corresponding monitoring systems was designed for on-site deployment.The moisture,viscosity,temperature,pollution and the correlation were analyzed through monitoring the oil index of the gearbox,and the monitoring data were compared with the offline detection data.The results show that the online oil monitoring system of the CRF circulation pump can truly reflect the real-time status of each indicator of the oil.The change trend of each indicator is conformed to the actual law of the oil,and the deviation from the offline detection data is within the normal range.The oil online monitoring system designed is highly reliable in the application of nuclear power circulating pumps and can be used as a basis for fault warning and diagnosis.