The most common defect found in the high pressure die casting (HPDC) process is gas porosity which significantly affects the mechanical properties of the final components. As it is known, the air entrapment of the liquid metal flow during the mold filling stage has a close relationship with the gas porosity distribution in die castings. The generation of the entrapped air is mainly due to the interaction between air and liquid metal in the die cavity. In the past few decades, extensive efforts have been made to develop numerical models for the simulation of mold filling. However, these efforts mainly focused on the single phase flow models which can only predict the liquid metal flow and assume that the effect of air in the die cavity is negligible. As a result, these models could not provide the quantity and location of entrapped air in die castings. Recently, several research results on tracking the entrapped air in the molten metal flow have been reported but some techniques still need to be improved further and much more researches are required. In this study, a liquid-gas coupled model is presented to simulate the air entrapment phenomenon during the HPDC mold filling process. The pressure of the entrapped air bubbles is solved at each time step.  The calculations of the liquid and gas phases are combined together through the pressure transfer. In order to verify the validity and stability of this model, a specially designed water analog experiment was carried out and compared with the numerical simulation results of the liquid-gas coupled model and the single phase flow model, respectively. The comparison shows that the liquid-gas coupled model has better prediction accuracy than the single phase flow model in tracking the entrapped air bubbles.