Combustion of agricultural residues for heat and electricity production is an efficient way to reduce amount of such waste source and emissions due to inappropriate treatment of them. However, unlike other biomass materials, agricultural residues often have high content of ash and certain ash forming elements with low meting temperatures, resulting in sintering and slagging during combustion processes. It complicates combustion of agricultural residues and restricts further utilization of this renewable energy source. In this work, sintering of a typical agricultural biomass cornstalk and effect of two phosphate additives, Ca(H2PO4)2·H2O and NH4H2PO4, were investigated by a combination of various techniques. Results indicated that the addition of Ca(H2PO4)2·H2O increased the initial melting temperature of cornstalk ash by 64–209 °C. The ash sintering behavior was greatly improved as a result of formation of high melting temperature K–Ca phosphates, Ca9MgK(PO4)7 and Ca2KP3O10 upon addition of the Ca(H2PO4)2·H2O. It is accompanied with reduced generation of low melting temperature K-rich silicates and the formation of large ash aggregates. Addition of NH4H2PO4 increased the initial ash melting temperature by 28–116 °C, but it had slight influence on the cornstalk ash sintering characteristics. The analysis revealed that, although the addition of NH4H2PO4 resulted in the formation of high melting temperature K2CaP2O7, certain amount of K phosphates and silicates eutectics were still formed, initiating and promoting sintering of ash from mixture of cornstalk and NH4H2PO4. Reactions between Ca(H2PO4)2·H2O and potassium salts under different conditions were also studied. It was found that reactions between Ca(H2PO4)2·H2O and potassium salts might already start as the temperature higher than 300 °C. K2CaP2O7, Ca2P2O7, Ca(PO3)2 and KCa(PO3)3 were identified as main products from these reactions and amounts of them considerably depends on the heat treatment temperatures and mixing ratios between Ca(H2PO4)2·H2O and potassium salts. Compared to KCl, K2CO3 had a higher reactivity toward the Ca(H2PO4)2·H2O under same study conditions. The current work shows the addition of Ca(H2PO4)2·H2O is a promising approach to alleviate the ash sintering issues and assure efficient and smooth combustion of cornstalk.