Abstract: Elliptic shape isolators under back pressure were studied with 3-D CFD(computational fluid dynamics) calculation.The results show that the shock train structure and the flow characteristic are symmetrical generally when the upstream Mach number is low under symmetrical free-stream and geometry,but the shock train structure and the flow characteristic become unsymmetrical obviously when the upstream Mach number is too high.As the ratio of major axis to minor axis decreases,the elliptic shape isolator's ability to support back pressure becomes better,but the Mach number and temperature of the isolator outlet are independent of the ratio of major axis to minor axis.As the upstream Mach number increases,the Mach number of outlet varies little once reaching maximum back pressure.