The invention discloses a method for monitoring ash deposition and slagging in a heated area. A to-be-measured heated area is selected and a high-temperature heat flow measurement point is arranged; ahigh-temperature heat flow meter is used for collecting a temperature signal and transmits the collected temperature signal to a data acquisition and processing system; calculation is performed to obtain different loads and corresponding heat flow densities qmn; an average load value and an average heat flow density value are calculated within a collection cycle T and the average load value and the average heat flow density value are transmitted to a data storage module; after data preprocessing and analysis, a maximum heat flow density value of the measurement point position is updated continuously and the average heat flow density value of the measurement point is compared with a maximum heat flow density value in a corresponding load to obtain a heat flow density difference value of the measurement point; and when the heat flow density difference value is greater than or equal to an ash deposition and slagging early warning value, an early warning signal indicating the serious ashdeposition and slagging of a boiler is sent out and the ash and slag removing measures of the boiler are taken. Therefore, a problem of selecting a reference value in slagging monitoring is solved;influences of flame pulsation and other factors are reduced; and the monitoring result is accurate and reliable.