Abstract: In China's mountainous areas, trucks have long-term downhill braking load problems. Due to the high cost of the hydraulic retarder and the severe thermal decay of the eddy current retarder when continuous braking, the retarder is promoted on trucks slowly. A new eddy current retarder that has a water-cooled coil was proposed in this paper. To solve the problem of unstable coil excitation due to high temperature caused by long-term braking, the heat-generating stator and excitation coil of retarder were both designed as water-cooled structures. The electromagnetic simulation method was used to analyze the static air-gap magnetic flux density before and after the retarder was assembled. The magnetic flux leakage was caused by the drive shaft. A method of reducing the magnetic leakage was proposed. The magnetic leakage ratio dropped from 21.2% to 3.4%. The problem of temperature field distribution of the retarder stator and excitation coil was solved by the finite element method. Combining with the transient electromagnetic field, the retarder braking torque under the coupling of the stator temperature field and the coil temperature field was solved. A prototype of the coil water-cooled eddy current retarder assembled on the rear axle of an 18-ton truck and the bench test and the vehicle test were carried out. The bench test results show that the experimental value of the retarder torque is in good agreement with the simulation value, and the error is within 9.7%. When the vehicle is fully loaded, the maximum braking deceleration of the truck is 0.61 m/s².