With the continuous improvement of the road condition, commercial vehicles get to be faster and more overloaded than before, which puts higher pressure on the vehicle braking system. Conventional friction braking has been difficult to meet the needs of high-power commercial vehicle. The auxiliary braking equipment will become the future trend for commercial vehicle. Hydraulic retarder is superior to secondary braking equipment. Previously hydraulic retarder research mainly focus on flow field analysis, the braking torque calculation, cascade system optimization and control methods for hydraulic retarder. The gas-liquid two-phase flow in working chamber is less researched. Based on this, this article discusses on the hydraulic retarder from two aspects. Firstly, this paper presents a block modeling method for hydraulic retarder system. Systemic model includes a rotor and a stator, the working chamber, the oil inlet, heat exchanger, fuel tank, the float member and the intake valve mechanism. Using the interface technology connects all the parts together for the subsequent numerical calculations of the flow field. Secondly, Based on the judgment of hydraulic retarder two-phase pattern using Tatid flow theory, choosing VOF two-phase flow numerical model and applying transient sliding mesh technique to deal with the strong interaction between rotor and stator, this paper analyzes the gas-liquid flow pattern and volume changes during the oil-filling process. The results show that, in the process of filling liquid, gas-liquid two-phase flow pattern can be divided into three patterns: smooth stratified flow, wavy flow and dispersed bubble flow according to the working chamber oil accounts.