With the aim to improve the hydraulic performance of liquid-ring pump, the three dimensional transient gas-liquid flow in liquid-ring pump was simulated by using the volume of fluid (VOF) method. The streamline, the distribution of velocity, press and the volume fraction were investigated. The secondary flow in pump was analyzed. The free interface of gas and liquid phase in pump was traced, and the relation between the shape of the free interface and the hydraulic performance of the pump was analyzed. The experimental research was carried out. The numerical results and the experimental results show good agreement. The numerical method of VOF is suitable for simulating the gas-liquid flow in liquid-ring pump. The free interface could be traced exactly and the hydraulic performance could be evaluated with enough accuracy, which pave the way for hydraulic optimization of liquid-ring pump.