With the aim to obtain the time-varying braking performance of emergency oil-filling process of hydraulic retarder, a numerical simulation model of flow channel was established based on transient CFD method. The inlet velocity, outlet velocity and a steady flow field result were calculated and applied as the boundary condition and initial condition. Then the transient simulation of emergency oil-filling process was calculated when the initial rotor rotating speed was 2640r/min. The pressure, the velocity and the turbulence kinetic energy distribution of internal flow field under different braking time were studied, and the time-varying braking torque was calculated by CFD post-processing analysis. Comparison of the experimental data and simulation results showed that the calculation tolerance of the braking torque was 12.7%. It is indicated that the simulation method and model are accurate and reliable, and the transient simulation method is more appropriate to predict the time-varying braking performance of hydraulic retarder.