To improve the braking power density of dual torus hydraulic retarder, the design method of blade had been studied. The tangent-arc blade design method was proposed base on the structural feature of bowed blade. On the design variables of blade wrap angles and working radiuses, the parametric modeling of periodic flow was achieved. With the Design of Experiment method (DOE), the design examples were adopted to study the braking performance on different blade parameters. Comparison with braking performance of the prototype showed that the numerical method was accurate and reliable, the relative braking torque error between simulation and experiment was within 5％, and based on the tangent-arc blade design method, the braking torque of different blade parameters had a large range of variation. With the appropriate blade parameters, the braking performance could be effectively improved.