Abstract: The thermal system simulation technology is used to solve the problem of high temperature of exhaust, short life of bag-filter and limited technical output of 330 MW circulating fluidized bed (CFB) thermal power unit boiler. The main factors of limited unit output is studied and the measures are presented. The results show that the decisive factor is the heat transfer performance of the air preheater for the problem of the limited technical output of the unit, and the low calorific value and moisture content of coal combustion have some impact on it. To some extent the use of coal variable-type operation can improve the technical output of the unit. The problem of limited load of the unit can be completely solved after the transformation of the internal thread tube air preheater. The temperature of exhaust gas reduces by 33.09 ℃, the thermal efficiency of boiler increases by 1.773 4% at full load, and the annual operation cost of coal is saved nearly 11.712 2 million yuan. The energy-saving economy is significant.