In this study, the co-combustion of Zhundong coal and NH4H2PO4 was investigated in a laboratory-scale tube furnace. The ash fusibility, combustion performance, and ash characteristics were quantified by experimental approaches. This study also used Gibbs free energy theory to analyze the chemical reactions. The results showed that NH4H2PO4 can extend the burning time of coal and reduce combustion intensity. Moreover, NH4H2PO4 significantly changed the composition of ash, which was caused by reactions between alkali/alkaline earth metals and NH4H2PO4, increasing the sodium retention ratio of the formed ash. In addition, the newly formed phosphate crystals (i.e., Mg2P2O7, AlPO4, and Ca-3(PO4)(2)) with higher melting temperatures contributed to the increase in fusion temperature of the formed ash, and the sodium-containing crystals (i.e., Na2CaP2O7 and NaMg4(PO4)(3)) contributed to the increase in the sodium retention ratio. Furthermore, NH4H2PO4 had a significant effect on the microcosmic structure, with ash showing more rough and sintering being inhibited.