Petroleum coke (petcoke), especially high vanadium (V) petcoke, has been produced increasingly because of the growing consumption of petroleum oil. Petcoke ash fusibility is closely related to ash slagging, which has an important impact on the clean and efficient utilization of petcoke. This study investigates a high V petcoke ash fusibility and its modification by coal blending and flux addition at reducing atmosphere. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to determine the crystalline minerals and surface morphology of high-temperature ash slag, respectively. The ash fusion process was predicted by thermodynamic equilibrium calculations. The results show that Shengli Oilfield petcoke (SL) has a high ash fusion temperature (AFT), whereas coal blending and flux addition can effectively decrease the AFT of SL, which may be ascribed to the mineral transformations during ash fusion process. The synergistic effects of high-melting vanadium trioxide (V2O3) and coulsonite (FeV2O4) result in the high AFT of SL. When SL blends with Shenmu (SM) coal and Qi coal (QC), the formation of low-melting andradite (Ca3Fe2Si3O12) and the decrease of V2O3 contribute to the progressive decline of AFT. When the addition of flux calcium oxide (CaO) reaches 10%, the low-melting calcium orthovanadate (Ca3V2O8) formed under reducing atmosphere decreases the AFT. With increasing CaO content, the content of V2O3 and FeV2O4 decreases while that of Ca3V2O8 and Ca3Fe2Si3O12 increases gradually, which further leads to a decrease in the AFT.