Silica (Si) and alumina (Al) elements are always considered to be the important inducements for ash sintering and slagging in the boilers. The effects of SiO2/Al2O3 (S/A) and (SiO2 + Al2O3) ratios on the synthetic petroleum coke (petcoke) ash fusibility are investigated in this work. Experimental methods including ash fusion temperatures (AFTs) tests, X-ray diffraction, and scanning electronic microscopy are applied to investigate the AFTs, the mineral composition, and surface morphologies of high-temperature ashes. Moreover, thermodynamic equilibrium calculations are also used to predict the ash fusion process. The results show that the AFTs of petcoke ash samples are closely related to the addition of S/A and (SiO2 + Al2O3). The dominant crystalline phases formed in high-temperature ashes with different S/A and (SiO2 + Al2O3) are gehlenite (Ca2Al2SiO7), quartz (SiO2), nickel orthosilicate (Ni2SiO4), and mullite (Al6Si2O13). The AFTs drop sharply with increasing S/A until it reaches 1.5, which may be ascribed to the gradual decline of high-melting Al6Si2O13. Then, the AFTs follow a slight increase within the S/A range of 1.5-3.0. As (SiO2 + Al2O3) increases from 20% to 60%, the AFTs show the continuous increase because of the formation of anorthite (CaAl2Si2O8) and Al6Si2O13 and the increased content of SiO2. The high-temperature ashes with the S/A and (SiO2 + Al2O3) of 1.5% and 20% have the higher molten extent. Particles in these ashes are almost in complete agglomeration and present the denser layer structure and more smooth surface.