Little work has been performed on the effects of H2O and HCI to particulate matter (PM). reduction by kaolin under oxy-coal combustion. To, determine the effect mechanism, a combustion experiment of pulverized coal mixed with kaolin was performed in a drop tube furnace. First, a low-pressure impactor was used as,a sampling device to investigate the particle,size distribution of PM when H2O and HCl were added during the first stage, and then a special sampling tube was used to collect the combustion products at 950 degrees C. Besides, the HSC Chemistry,6.0 thermodynamic software was also used for equilibrium calculations of sodium species in the drop tube furnace. The results indicate that H2O can enhance the absolute amount of PM0.2 captured by kaolin through promotion of the generation and diffusion of sodium hydroxide during oxy-coal combustion. However, the increase in PM0.2 generated from Cog in the presence of H2O overrides the increase in the PM, captured by kaolin; thus, the ratio of PM0.2 captured by kaolin to the total PM0.2 produced decreases during oxy-coal combustion: HCI can suppress the capture of sodium by kaolin and inhibit the efficiency of PM, reduction by kaolin during oxy-coal combustion.