Abstract: A new method of on-line calculation of heat efficiency of circulating fluidized bed boiler (CFB Boiler) was proposed. Based on some measurable operation parameters, including pressure, temperature and the flow rates of main steam, water and primary and secondary air, which were collected online, input energy and the utilization of energy in effect could be deduced respectively, and then heat efficiency could be calculated. Using the plant-site data from a 135 t/h CFB boiler unit in Fujian, the online calculation of heat efficiency was simulated and the result of industrial simulation indicated that this method was accurate and effective. Furthermore, orthogonal experiment was performed to analyze seven factors which contributed to heat efficiency of CFB boiler. Because it is hard to get corresponding data on the orthogonal layout, a Levenberg-Marquardt (LM)_back propagation (BP) algorithm-based predictive model was established by using the data collected from plant-site and on-line calculation model. The result of orthogonal experiment indicated that air/coal ratio, main steam pressure and boiler firebox temperature were key factors. The result was validated by the industrial optimization experiment. This method will help on-line optimal operation of the CFB boiler.