In order to study thermal calculation and optimization scheme of thermal power boiler, through the flexible use of equilibrium analysis method, the practical problems brought about by too high exhaust gas temperature of thermal power boiler are studied. Through the thermodynamic calculation, the thermal calculation simulation is carried out under 95%, 85%, and 60% three kinds of operational loads for boiler after after low temperature reheater retrofit optimization and heating surface increase. In addition, it is compared with the calculation parameters under the corresponding condition before the transformation. The results show that the increase of low temperature reheater area can increase the low temperature reheater heat absorption, and make the reheat steam humidity improved significantly. At the same time, the flue gas baffle is adjusted and the gas ratio on the side of overheating is increased. As a result, we can effectively alleviate the serious uneven reheat and super-heat side baffle opening for a long time. However, low temperature reheater outlet flue gas temperature and economizer inlet gas temperature can be significantly reduced because of the transformation and optimization of low humidity reheater. It is conclude that the final temperature of flue gas has not been significantly reduced, which only completed part of requirement for the exhaust gas temperature reduction.