Currently, the most widely used method to calculate boiler efficiency is based on wet basis oxygen which is measured by online instrument, rather than on dry oxygen. Consequently, it often results in the deviation of calculation results, as well as bad adaptability to different kinds of coal. In this study, an online calculation method of boiler efficiency as deducted based on the wet oxygen, and validated by typical soft coal and lignite. It is revealed that the results of this calculation model is more close to that of standard calculation method by elementary analysis. Moreover, the online calculation method can exhibit a better performance of coal adaption, which can be used as a method to guide for the better optimization of utility boiler combustion.