Ash fusion behavior is closely associated with ash-related problems including fouling, sintering, and slagging, which results in a negative effect on the utilization of petroleum coke (petcoke). Petcoke ash contains high levels of vanadium (V), nickel (Ni), iron (Fe), and calcium (Ca). The chemical composition of ash plays an intrinsic role in determining ash fusibility. To better understand the modification mechanism of the ash fusion temperatures (AFTs), this study investigates the influences of ash composition (CaO, Fe(2)o(3), and NiO) on the synthetic petcoke ash fusibility from the perspectives of ash composition change and temperature rising. The AFTs of synthetic ash sample's were identified by the ash fusibility tester. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were applied to explore the relationships between the experimental AFTs and the variation of mineral composition and microstructure of high-temperature ash slag. Moreover, the ash melting process was predicted by the SiO2 Al2O3 V2O5 CaO Fe2O3 NiO system based on the FactSage modeling. The results show that the AFTs of petcoke are closely related to the ash chemical composition. As the CaO and Fe2O3 content increases, AFTs exhibit continuous decline, while first decreasing slightly and then increasing with the increasing NiO content, which is ascribed to :the different mineral transformation behaviors of high-temperature ash slag. The dominant crystalline minerals formed in high-temperature ash slag with different CaO, Fe2O3, and NiO content are anorthite (CaAl2Si2O8), orthosilicate (Ni2SiO4), calcium pyrovanadate (Ca2V2O7), and quartz (SiO2). Fe may form Fe-beating amorphous matter with other minerals. The synergistic effect between high-melting Ni,SiO, and low-melting Ca2V2O7 may contribute to the variation of AFTs, which was well validated through thermodynamic equilibrium calculations.