In order to improve power density of vehicular hydrodynamic transmission components, with the aspect of structure design of cascade system, a blades design method for flat circular circle was put forward. Based on the analysis of constraint conditions and design streamline of traditional circular circle, the flatness ratio could be changed from 0.31 to 0.19 with only three parameters of effective diameter D, minimum diameter d0 and width limit Wlim. Besides, the program was rapid and convenient. Compared with traditional hydrodynamic components, the working volume obtained by flexible flat circular circle could be compressed to 40.9% with the same maximum diameter D and minimum diameter d0. The power density of hydrodynamic components was improved effectively and meanwhile traditional circular circle design method was simplified distinctly.