Abstract: Calculating the boiler thermal efficiency has become an indispensable task to evaluate and inspect the economy and energy-saving potential in coal-fired power plant at present. When a parameter changes, the existing correction method of boiler thermal efficiency is inconvenient to analyze its influence to the boiler thermal efficiency, and it is also inconvenient to analyze the reason and location which causes the reduction of the boiler thermal efficiency in operation. A new correction method which combined differential deviation method with first order Taylor formula was proposed to revise the various losses and boiler thermal efficiency, and comparison and analysis of correction calculation method were also conducted between the innovative correction method and ASME PTC4—1998. Taking a 1025 t/h boiler for example, various losses and boiler thermal efficiency were calculated under two different test conditions. The results show that the difference of the losses and boiler thermal efficiency calculated by differential modification method and ASME PTC4—1998 is tiny after modification, so the new method is convenient to guide operation for the boiler and also can be applied into the engineering practice.