Abstract: Alkali slagging and agglomeration during biomass combustion utilization restrict the large-scale utilization of biomass resources. In order to alleviate the problem of biomass slagging, the contents of different forms of the potassium contained in cotton stalk are determined, and the effects of potassium on slagging characteristics of the cotton stalk are investigated. The influences of calcium carbonate, calcium phosphate and ammonium dihydrogen phosphate on the melting characteristics of cotton rod ash are studied, and the anti-slagging mechanisms are explored. The results show that most of potassium can be removed from the cotton stalk by fractional extraction. The melting point of the cotton rod ash can be significantly increased by water extraction, but decreased after ammonium acetate extraction, without obvious effects after hydrochloric acid extraction. Calcium phosphate can fix the potassium in cotton stalk ash in the ternary salt system of K-Ca-P and increase the characteristic temperature of ash melting, which has a good anti-slagging effect. Ammonium dihydrogen phosphate has a good effect on potassium fixation but does not improve the melting characteristics of cotton rod ash. The calcium carbonate shows weak effect on the potassium but increases the sintering temperature and ash melting characteristic temperature of the cotton stalk ash. The slagging resistance of the three additives is calcium phosphate, followed by calcium carbonate; and ammonium dihydrogen phosphate is the weakest.