Self-recirculation casing treatment can significantly improve the aerodynamic performance of the supercritical carbon dioxide centrifugal impeller in small flow rate region, but the improvement is not obvious near the large flow rate region. Therefore, the coupling effect between the self-recirculation casing treatment and the key parameter of the impeller is considered, and the coupling optimization of the casing treatment geometry and the impeller blade sweep angle is carried out to achieve a comprehensive improvement of the impeller performance. After the coupling optimization, the efficiency of the impeller is increased by 3.51%, 2.60% and 4.43% respectively under the large flow rate condition, the design condition and the small flow rate condition. The mechanisms of the coupling optimization for stability and efficiency enhancements are as follows. Under the large flow rate condition, the flow incidence angle of impeller is improved, the subcritical zone inside the impeller is reduced, then the condensation is suppressed and the flow capacity of the impeller is improved. Under the design condition, the recirculation flow of casing treatment is increased, more low-energy fluid near the shroud tip is removed, and the flow field structure downstream of the impeller is improved. Under the small flow rate condition, the internal blockage of the impeller is effectively reduced, the flow stability of the impeller is enhanced, and the mixing loss caused by the recirculation flow is improved, so the impeller efficiency is improved.