Abstract: The low temperature economizer and air heater combined cycle system can improve the efficiency of the power plant boiler effectively and avoid blockage of the air heater system. Its application on the boilers in some power plants has been proved positive. However, the problem of uneven flow distribution is common in practice, which may result in the cracking of the heat exchanger tubes in winter in northern China, and harm the operation of the equipments. To better solve this problem, based on the mechanism of flow distribution of the working fluid in the air heater, this paper proposes a numerical calculation model on FLUENT software platform and a hydrodynamic calculation model on Visual Basic 6.0 platform. Results from the two models are compared. The influence of the heat load, the gravity and velocity of the working , the spacing between the heat exchanger tubes, and the location of collecting headers on the flow distribution in the heat exchanger tubes is analyzed. It is found that the uneven flow distribution is aggravated with the increase of the working fluid velocity. As the elevation of the collecting headers increases, the working fluid gravity plays a more important role and the uneven flow distribution is also aggravated. However, the increase of the velocity incurs a stronger forced flow in the collecting headers, which weakens the effect of the gravity. The coupled effect of the heat load and the gravity is the major cause of the uneven flow distribution in the air heater. Based on the calculation results, several schemes are proposed to prevent the uneven flow distribution. The results of this paper provide some solid technical backup for a better design of an air heater and solve the problem of tube cracking inside the air heaters due to low temperature.