A zero inlet swirl turbine rotor (ZISTR) works with axial inlet gas flow directly exhausted from a combustion chamber without any upstream vanes. The suction side trailing edge shock of the ZISTR generates a significant amount of aerodynamic loss. A shock loss reduction method is presented for the ZISTR. The principle of the method is to replace the single shock by double shocks. For minimizing the shock loss, the strength of the double shocks is set to be equal. This is based on a design criterion that multiple shocks should be with equal strength to achieve the minimum shock loss. This design criterion has been widely used for supersonic inlet design, but it is first extended to the transonic turbine design in the present article. A shock model, using aerodynamic relations of oblique shock, is put forward to quantitatively determine the shape of the concave suction-side profile. Numerical simulations indicate that the concave suction-side profile induces a new shock upstream of the suction-side trailing edge shock, and these two shocks are with nearly equal strength in the improved ZISTR. The shock loss of the improved ZISTR is obviously reduced. The concave tip profile would redistribute the blade load on the tip and increase the tip leakage flow loss. Hence, it is suggested that the concave profiles should be designed between 0% and 80% blade span, and the tip profile should keep unchanged. The improved ZISTR with an unchanged tip profile achieves an improvement of turbine efficiency by 0.8% at the design condition.