The organic Rankine cycle is widely used in industrial waste heat, engine waste heat and other waste heat recovery applications, and as a key component of the system, it affects the efficiency and output power of the system. In this paper, a centripetal turbine is designed for the organic Rankine cycle, using vehicle exhaust gas as the heat source. Numerical simulations are performed to analyze the effect of the ratio of the number of guide vane blades to the number of impeller blades (vane number ratio) on the turbine performance and flow field. The results show that the effect of the number of impeller blades on the turbine entropy efficiency, the average exit velocity and the temperature of the guiding grate becomes less and less as the ratio of the number of blades increases. The optimum turbine performance is obtained when the number of impeller blades and the ratio of the number of blades are 17 and 1.5882, respectively, and the expansion performance of the guide impeller is improved and the isentropic efficiency of the turbine is improved by 3.84% compared with the preliminary number of blades.