Abstract: To investigate the temperature distribution characteristics of vertical membrane waterwall during combustion under various boiler conditions, the experiments of secondary air distribution in the upper and lower furnace including C-layer secondary-air on the arch and F-layer secondary-air in the lower furnace, fuel-lean air were carried out in a 600 MW subcritical down-fired boiler. The results indicate that the temperature distribution characteristics of vertical membrane waterwall, especially the uniformity of wall temperature distribution in width direction of boiler, are closely dependent on the secondary air distribution. To ensure the safety of water wall, through the analysis of the temperature deviation, the peak temperature and the average temperature of waterwall, the proper adjustment of the secondary air distribution could be determined and proved this relationship to be true under various loads. The research results show that the F-layer secondary-air could control the flame shape during combustion, and the inconformity of the secondary air distribution in the front and rear walls would lead to the flame deflection and the peak temperature of waterwall. The C-layer secondary-air on the arch could lengthen the fuel-rich coal/air downward distance. While the excessive C-layer secondary-air could easily cause the local combustion concentration and the nonuniformity of wall temperature distribution. Excessive fuel-lean air is favorable to ignite, but cause high heat load of water wall surrounding the burner. Shut down the fuel-lean air as much as possible on the basis of ensuring the ignition and steady combustion, which could be good for both the furnace flame fullness and the even waterwall heat absorption.