A multi-objective optimization of a coupled casing treatment (CCT) for an axial transonic compressor is performed in this study. The coupled casing treatment is the basis axial slots with a circumferential groove located at various positions along the slots. During the optimization stage, five important parameters to control the geometry are used as the optimal variables. The stall margin and the peak efficiency are selected as the optimal objectives. Non-dominated sorting genetic algorithm II coupled with radial basis function (RBF) approximation is used to search for Pareto-optimal solutions. Then, four optimal configurations are selected from Pareto-front for further analysis. As shown in the simulation results with and without the coupled casing treatments, the leakage flow is reenergized and the blocking region near the blade leading edge at rotor tip is decreased by the use of these structures under the low flowrate condition, which is the main reason for stability enhancement. Besides, a coupled casing treatment with the groove settled near the end of the basis slots have the potential to generate more injection flow and extend the operating range of compressor further.