Abstract: In this work, based on theory of the turbulent jet and the turbulent boundary layers, the turbulent fluctuation properties in the mixing section of the round injector have been researched. From Reynolds equations the basic equations and the momentum integrating equation of turbulent flow field to cylindrical coordinates in the round injector have been presented. The approach of designing the optimal injector has been developed, and the formula of the optimal designing of the injector have been put forward.