The ash melting temperature of X coal with high ash melting temperature was reduced by adding calcium-magnesium compound flux (WCaO/WMgO=1). In the process of simulated coal gasification, the ash and slag were prepared, and the existence form and evolution process of oxygen, silicon, aluminum, calcium, magnesium at different temperature were analyzed by X-ray photoelectron spectroscopy (XPS). Calcium-magnesium compound flux can reduce ash melting temperature and mainly affect structure change of silicon, aluminum and oxygen. The change of the coordination mode of aluminum and oxygen was shown, namely aluminum-oxide tetrahedrons ［AlO4］ and aluminum-oxide tetrahedrons ［AlO6］ varied with temperature changes. The addition of Ca2+ and Mg2+ can destroy the silica chain, making bridging oxygen silicon change into non bridge oxygen silicon; Bridged oxygen bond fracture was broken and non bridging oxygen bond was formed in oxygen element structure. Combined with the Factsage thermodynamic analysis software, the high temperature phase equilibrium composition of coal ash was analyzed after adding calcium-magnesium compound flux, and the fluxing mechanism of flux was studied from the change of mineral structure. The results showed that Ca2+ and Mg2+ were easy to combine with the non bridging oxygen bonds in the silicon-oxide and aluminum-oxide tetrahedrons and aluminum-oxide octahedrons to produce low melting temperature feldspar minerals and magnesia minerals. Therefore the ash melting temperatures were reduced.