At present, in the calculation process of the braking flow field of hydraulic retarder, the flowresistance and voltagedrop effects of heat exchanger core are ignored, resulting in inaccurate setting of calculation boundary conditions. Therefore, based on model reconstruction technique of threedimensional scanning reduction, by taking the fullrunner selection scheme, using RNG twoequation model and pressurebased PISO algorithm, considering the flowresistance and voltagedrop effects of heat exchanger core, an overall numerical computation was conducted on the braking flow field of hydrodynamic retarder VR120 from VOITH by using computer fluid dynamics (CFD) technique. The curves of braking torque and speed characteristics were derived, and verification analysis was conducted on flowresistance and voltagedrop effects of the heat exchanger core with using of nephogram of flow field pressure.The results show that the braking torque presents a quadratic growth trend as the speed increases; the heat exchanger core has significant flowresistance and voltagedrop effects, which is nonnegligible flow field boundary conditions, and fullrunner computation method is necessary.