Abstract: In order to find out the causes of heat loss of circulating fluidized bed( CFB) boiler unit,the thermal performance of CFB coalfired boiler with rated load of 75 t/h was tested. In order to compare with the measured method,a new idea of obtaining the thermal efficiency of CFB boiler based on Aspen Plus model method was proposed,that is,by using Aspen Plus process simulation software to model and calculate CFB boiler. In the experiment,three operating conditions of low load,full load and high load were selected,and the heat losses was calculated by the anti-balance method,and then the effects of different operating conditions on the thermal efficiency of the CFB boiler were explored,the distribution of unburned carbon( UBC) in fly ash and slag under different operating conditions was analyzed.By modeling the four sub-processes of coal pyrolysis,coal combustion,flue gas-fly ash separation and heat exchange of CFB boiler,and based on the data of inlet and outlet flow unit temperature,pressure,flow rate,etc. in heat exchange equipment at all levels of the boiler,the heat loss,the heat efficiency and the concentration of flue gas at the outlet of the boiler were calculated under the condition of full load( working condition 2). The accuracy and reliability of the model were verified by comparing the measured data with the simulation results.The results show that the model method is in good agreement with the measured data and can accurately predict the composition of flue gas at furnace outlet. Through the comparison of the heat loss and thermal efficiency of boiler,it is found that the measured result of exhaust heat loss q2 is 7.75%,and the model result is 6.48%.The measured result of solid incomplete combustion q4 is 3.72% and the model result is 3.17%.The relative error between two methods is small,indicating that Aspen Plus modeling can be used to accurately predict the heat loss of smoke exhaust and the heat loss of incomplete solid combustion. The thermal efficiency of the boiler calculated by the model is88.66%,the measured thermal efficiency of boiler is 87.426%,and the relative error is only 1.41%,the calculation results of thermal efficiency and heat loss is extremely close between the measured method and the model method,which can verify the veracity and reliability of the modeling idea and method,and the feasibility of CFB boiler thermal efficiency calculation based on Aspen Plus model is also verified.There are some problems in the operation of the boiler under the three operating conditions,such as high flue gas temperature,high carbon content of fly ash,and low actual thermal efficiency that does not reach the boiler design value. The content of UBC in fly ash after coal combustion is relatively high,ranging from 13.28% to 16.40%,while the content of UBC in slag is low,ranging from 2.92% to3.39% under three operating conditions,the heat loss of boiler exhaust flue gas between 7. 64%-7. 93%,the heat loss of solid incomplete combustion between 3.72%-4.69%,the boiler thermal efficiency is between 86.14%-87.43%,in which η2>η3>η1.It shows that it is feasible and reliable to use Aspen Plus to model CFB boiler for thermal calculation.