To solve the problems of insufficient braking torque at low speed of existing hydraulic retarders, low braking torque at high speed of eddy current retarders, and severe thermal degradation of mechanical friction brakes, this paper proposes a novel Mechanical-Hydraulic composite Brake (MHB) for the first time. Firstly, the structure and working principle of MHB are introduced. Secondly, a 3D simulation model is established by applying CFD technology to obtain the distribution of pressure and velocity fields, and the braking performance of MHB is predicted by the numerical simulation method. Subsequently, the braking performance of MHB is verified through experiments. Finally, the advantages of MHB were analyzed by comparing with existing retarders. The results show that the mechanical braking part of MHB can provide 8000 N·m torque, and the hydraulic retardation part can provide 1450 N·m torque at 2500 rpm. The braking performance of the MHB hydraulic retardation part is improved by 15% ~ 35% compared to the existing retarders.