Abstract: Particle agglomeration, fouling on the heating surface and other issues are closely related to the alkali metals during biomass thermo-chemical conversion. The potassium fixation ability and ash fusing characteristics in the combustion of rice straw (RS) added with ammonium dihydrogen phosphate (NH₄H₂PO₄) were studied in a 21%O₂/79%CO₂ atmosphere. The reaction residue from the experiment in a drop tube furnace was analyzed with inductively coupled plasma atomic emission spectrometer (ICP-AES), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The potassium retention ratio of co-combustion of RS and NH₄H₂PO₄ is higher than that of RS alone, especially at 900℃, and the retention ratio increases from 14.65% to 68.79%. The reaction between NH₄H₂PO₄ and alkali metals in rice straw leads to the formation of KPO₃ at 700 ℃ and K₂CaP₂O₇ at 900℃, which is a high melting point compound. SEM analysis on residue indicates the agglomeration is inhibited, and the ash fusion temperature of RS is increased by adding NH₄H₂PO₄. These results can be helpful to the research and development of high-alkali biomass clean utilization process.