In this study, the structure and working principles of an eddy current retarder acting as an auxiliary brake set is introduced in detail. Based on the principle of energy conservation, a mathematical model was developed to design a retarder whose nominal brake torque is 1,900 $N{cdot}m$. According to the characteristics of the eddy current retarder, an exclusive test bed was developed and used for brake performance measurements. The main technical parameters, such as the brake characteristics, temperature characteristics and power consumption, were measured with the test bed. The test data show that the brake torque of the eddy current retarder obviously decreased in the continuous braking stage and that there is a certain amount of brake torque in the normal driving state because of the remnant magnetism of the rotor plate. The mathematical model could be used to design an eddy current retarder. The exclusive test bed could be used for optimization of an eddy current retarder as well as for R&D of a series of products.