Considering the remedy of cofiring on biomass slagging and less concerns on different biomass cofiring, the effects of blending ratio of straw (a representation of agricultural biomass) and sawdust (a representation of woody biomass) during cofiring, as well as CaO, MgO, SiO2, and K2O additives, on ash fusion characteristics are studied using sintering tester, XRF, XRD, and SEM. The ash fusion temperatures (AFTs) of straw are significantly lower than those of sawdust due to higher K2O, SiO2, and Cl, and lower CaO and MgO contents in the straw ash, consequently, the AFTs decrease with increased straw/sawdust blending ratio because of the formation of low-melting K2SO4, NaCl, CaSi2, K2MgSi3O8, Na2Si2O8, and K2Si4O9. However, attributed to the dominating straw ash content and components in mixtures, the AFTs keep unchanged with increasing blending ratio above 3:2. SiO2 and K2O improve the generation of K2O center dot nSiO(2), KCl, and K2SO4 with low-melting temperature, thus lowering AFTs and worsening slagging; Whereas CaO and MgO improve the formation of high-melting silicates such as Ca2SiO4 and CaMgSiO4, thus rising AFTs and weakening slagging. There exists a transformation from chemical reactions to physical addition with increased dosages of CaO and MgO. To ease fusion slagging of agricultural residues during combustion, the cofired woody biomass should account for at least 40 wt %. These general results are useful for biomass-fired power plants arranging the cofiring of woody biomass and agricultural residues.