Objective To preliminarily clarify the effect of parameters on coatings in supersonic plasma spraying and to obtain the optimal process parameters. Methods Al₂ O₃ ceramic coatings were prepared by the supersonic plasma spraying technique. The microstructure and properties of Al₂ O₃ coating were characterized and the effect of spraying parameters such as spraying current, pressure of plasma gas and spray distance were also discussed. According to the response surface methodology, the optimization experiment with three factors and three levels was designed and the regression equation was established between the influencing factors and the response value, thickness. Results When I = 260 A or P_air = 0. 55 MPa, the coating was a dual-phase composite structure consisting of α-Al₂ O₃ and γ-Al₂ O₃ . When I = 340 A or P_air = 0. 35 MPa, the coating was almost fully deposited into the γ-Al₂ O₃ phase, substantially containing no unmelted particles. Change in spraying distance had no significant effect on the phase composition of the coating. When I or d increased or P_air reduced, HV tended to increase. Conclusion In the experimental range, the optimal process parameters for microhardness of supersonic plasma spray were P_air = 0. 35 MPa, I = 340 A and d = 230 mm.