Abstract: Due to the harsh service environment, wind power hub castings are required to have dense structure and excellent mechanical performance. The secondary oxidation slag formed during the casting filling process is prone to cause local stress concentration, which seriously affects its service performance. Therefore, predicting the location of secondary oxidation slag inclusion and avoiding it has become the key to process design. The casting process design of wind power hub castings was completed and the filling process of wind power hub castings was simulated with the help of Flow-3D software. The influence of filling speed and ingate position on filling stability were analyzed, and the possible formation parts of secondary oxidation slag were predicted by tracking the concentration of secondary oxidation slag, and the process optimization scheme proposed as following: semi-closed bottom gating system, with horizontal variable cross-section trapezoidal runner, three circular cross-section ingates positioned directly opposite the large arc bosses, the cross-sectional area ratio of F_ingate∶F_runner∶F_sprue being 1 ∶ 2.4 ∶ 1.7, pouring temperature of 1360 ℃, and pouring speed of 1.3 m/s. Simulation of the mold filling process using the optimized process scheme reveals that the concentration of secondary oxidation slag at the corners of the large plane of the blade hole and the lower surface of the casting are reduced from 500 kg/m³ to below 100 kg/m³, while the concentration on the upper surface of the casting exceeded 500 kg/m³. Most of the secondary oxidation slag is transferred to the upper surface of the casting. These indicate that the optimized casting process scheme is reasonable, which is verified by experimental specimens.