According to the actual operation data of 1 000 MW ultra supercritical boiler, the slagging characteristics of the boiler are studied and analyzed. The prediction models of slagging characteristics based on the coal ash characteristics are established, i.e., prediction models of single index and prediction models of comprehensive index. The prediction model can predict the slagging characteristics of the boiler simply and quickly, but the accuracy and reliability of the prediction model will decrease when the boiler is burned with coal quality unevenness or mixed coal. At the same time, according to the real-time monitoring data of boiler operation, the on-line monitoring model of slagging characteristics based on furnace cleaning factor is established. Through the inlet and outlet steam parameters of the heating surfaces after furnace outlet, as well as the economizer export flue gas temperature and other relevant parameters, the calculation method of counter balance is used to calculate furnace exit gas temperature, and then the furnace average heat effectiveness coefficient and furnace cleaning factor are obtained. Finally, the boiler slagging characteristics are determined according to the cleaning factor. The model can not only realize the on-line monitoring of boiler slagging characteristics by collecting the real-time data of boiler operation,but also predict and judge the slagging characteristics of the boiler when the coal is not uneven and the mixed coal combustion is carried out.