The behavior of ash fusion at high temperatures plays a key role in the stable operation of gasifiers. This study investigated the effect of synthetic mineral compounds on the transformation and ash fusion temperatures (AFTs) of petroleum coke (petcoke) ash in a steam atmosphere with the variation of the SiO2/Al2O3 (Si/Al), Fe2O3/CaO (Fe/Ca), and V2O5/NiO (V/Ni) ratios and temperature. Thermodynamic equilibrium calculation was also applied to simulate the ash-melting process in petcoke gasification. The results show that the dominant crystalline phases in petcoke ash at high temperature are CaAl2Si2O8, FeAl2O4, and FeV2O4. The increase of temperature is conducive to the formation of low-melting minerals, such as, CaAl2Si2O8 and FeAl2O4. High Si/Al is beneficial for the reduction of AFTs because high melting point minerals partly convert into slag and the low melting point mineral phase of CaAl2Si2O8 transforms largely into slag. AFTs could be reduced at an Fe/Ca ratio of 0.5 when the content of slag in the ash reaches maximum and there are plenty of the low-melting minerals (CaAl2Si2O8). A high V/Ni ratio was not conducive to the suppression of AFTs. The effect of Si/Al and Fe/Ca could effectively improve the AFTs of petcoke ash. A high ratio of Si/Al and the ratio of Fe/Ca at 0.5 were beneficial to the ash fusibility. AFTs under gasification atmosphere conditions have been predicted by FactSage.