Abstract: The inlet flue gas temperature of SCR denitrification unit in the coal-fired power plant usually does not meet the requirement at low load. In some power plants the bypass heating method is applied, however, it is not effective to raise the flue gas temperature as expected and the problem of large deviation of flue gas temperature still exists. Taking a 300 MW unit in Power Plant A as an example, based on the field investigation, the problems are analyzed and diagnosed by numerical simulation as follows: (1) unreasonable structure of bypass flue due to the original structural defects; (2) lack of diversion elements installed in bypass flue duct; (3) unreasonable location of bypass access; (4) insufficient depth of bypass access. Therefore, on the basis of summarizing the highlights of design and operation of by-pass flue for denitrification and temperature regulation, the design of a 330 MW unit in Power Plant B is optimized by means of numerical simulation, and then the field tests are carried out to evaluate the effects of flue gas heating. The results demonstrate remarkable effectiveness of the retrofit project on flue gas bypass heating in Power Plant B with minor deviation of flue gas temperature.