Abstract: In this study,the fly ashes are selected from two typical biomass power plant dust collectors,the composition of fly ashes and their loss on ignition characteristics at high temperatures have been studied through the combined experiments of TG-FTIR,XRF,EDS,XRD and other analytical methods.The study shows that weight loss mechanism of biomass fly ash at high temperature is more complicated,and the following chemical processes will occur when the fly ash are heated in air:unburned carbon is burned out and the MgCO₃ is decomposed below 600℃and the decomposition of CaCO₃ occurs at 600℃-750℃.The evaporation of alkali chloride starts slowly at 380℃，and the evaporation speed increases rapidly higher than 750℃and completely evaporation of alkali chloride happens until 950℃.The vitrification reaction of minerals and the evaporation of alkali sulfates occurs above 950℃.According to the study on weight loss characteristics,the research reveals that the results of carbon contents in biomass fly ash tested by current DL and ATSM standard are not accurate.Based on loss on ignition mechanism of biomass fly ash,this paper proposes a more accurate test method for carbon content analysis,that is the difference value of the ignition losses reacted at 600℃under air and nitrogen.