Efficient utilization of the low-rank coal has been a headachy problem, especially when firing the high alkali-containing coal in a typical pulverized fuel boiler, where severe slagging and fouling originating from the alkali metal vapors may occur. The additives injection technology has been proved to be a promising method in combating these problems. In this study, the alkali capture mechanism of kaolin was investigated by burning a kind of high-sodium lignite in a laboratory-scale drop tube furnace. The effects of kaolin content, reaction temperature, and particle sizes of both kaolin and fuel on the sodium capture efficiency of kaolin were also investigated. It was found that kaolin could chemically adsorb NaCl, the primary sodium species proved in the flue gas, to form high-melting sodium aluminosilicates such as nepheline and albite, and the nepheline-forming reaction dominated the sorption mechanism. More kaolin addition led to more sodium fixed into the ash. However, the promotion was not that pronounced in high kaolin dosages. The sodium capture efficiency decreased as temperature was increased or larger kaolin particles were injected. Effect of the coal size on the sodium capture efficiency could be neglected in the tested size range. The sodium retention with 6 wt % kaolin addition of the fuel at 1200 degrees C could attain 70% of the total sodium in the combusted coal, which can considerably reduce the ash-related problems and facilitate the safe firing of high alkali coal in boilers.