Abstract: To overcome the deficiency of delayed heat release signals from circulating fluidized bed (CFB) boiler, a method was put forward for calculation of the heat release and carbon residue by using easily measured signals of the combustion process. The specific way is to firstly divide the combusion system into dense phase area, dilute phase area and adiabatic cyclone separator area, then calculate the combustion share in adiabatic cyclone separator area based on materials mass and heat balance model, and finally build up carbon residue model by mechanism analysis and heat release model according to carbon residue and air volume. Based on experimental data of a 300 MW CFB boiler, and using extended Kalman filter technology, the in-furnace carbon residue and heat release were calculated respectively at loads of 285 MW and 200 MW, of which the results were compared with relevant literature data. Results show that the calculated results basically agree with relevant literature data, proving the model to be accurate and reasonable.