Abstract: During the operation of a power plant, the differential pressure of air preheater increased, and the differential pressure was much greater than the design value. By analyzing the reasons of coal type, soot blowing steam, exhaust gas temperature and ammonia injection, the reasons for the increase of differential pressure of air preheater were obtained. After the reason was clarified, corresponding technical measures were taken. After the technical transformation and optimization adjustment of precise ammonia injection, the problem of large differential pressure of air preheater was completely solved. The cause analysis and solution of large differential pressure of air preheater can be used as a reference for other units.