Ash fusion characteristics of biomass have significant effect on slagging. In this work, the effects of ash fusion characteristics on slagging have been studied by ash fusion experiments and CFD modelling. Based on the basic ash composition of biomass, the mixture of silicon dioxide, calcium oxide, potassium oxide, and aluminum oxide have been selected as simulated ashes for ash fusion characteristics investigation. The results indicate that deformation temperature decreases with increased potassium oxide. High content of calcium oxide and silicon dioxide increase deformation temperature for the skeleton effect, respectively. The reactions of potassium oxide, calcium oxide and silicon dioxide lead to low melting products and decrease deformation temperature. Aluminum oxide increases deformation temperature by forming Si-Al-K compounds while K content is high and decreases deformation temperature while Ca or Si content is high due to the Si-Al-Ca compounds. On basis of the experimental results, an ash particle adhesion model has been developed using the corresponding characteristic temperatures of adhesion. Combined with the deposition model of inertial impaction, a CFD modelling study of ash deposition on heating surface has been performed. For a kind of cotton straw ash with low melting temperature, the modelling results indicate that adhesion of molten ash plays a major role during slagging. The accretion rate of molten ash adhesion accounts for 85% of the total accretion rate. For a kind of corn straw ash with high melting temperature, the proportion is only 37%. Compared with the actual slagging during biomass combustion, the modelling results can reflect a similar slagging situation on the surface of tube.