Abstract: The common clay mineral of kaolin was selected as an additive into Zhundong coal during combustion. Sodium retention rates and coal ash fusion temperatures were investigated at different blending ratios and temperatures. XRD and ternary phase diagram were used to identify mineral transformations at high temperatures. The results show that the sodium retention rates increase rapidly with blending ratio from 0~2%, slowly from 2%~5% and decrease slightly with increasing temperature. The ash fusion temperatures of coal blended with additive initially decrease slowly, then decrease rapidly and finally increase with increasing blending ratio, reaching about 1 200 ℃ at 3%. The results of XRD and ternary phase diagram analysis show that the change of ash fusion temperature is due to the mineral content changes of rankinite, gehlenite and anorthite. The lowest ash fusion temperatures at the blending ratios of 3%and 4% are mainly caused by the low temperature eutectic phenomenon. The sodium retention rate reaches more than 60% and ash fusion temperature reaches about 1 300 ℃ at blending ratio of 2%, which is conductive to solid-state slag-tap boiler. The ash fusion temperature is about 1 200 ℃ at blending ratio of 3.0~4.0%, which is suitable for liquid-state slag-tap boiler.