Abstract: Most of the ultra deep gas wells in China are high temperature, high pressure and high sulfur. The problems such as unclear blowout mechanism of acid gas wells and difficult wellbore pressure inversion lead to great challenges in well control. In order to reveal the causes of acid gas well blowout and clarify the influence of different acid gas content on annulus fluid flow, this paper considers the influence of solubility and phase state change on the physical properties of acid gas, uses the method of quasi critical parameters instead of critical parameters to analyze the variation laws of gas density and solubility along well depth, and establishes an acid gas annulus flow model with dissolution phase change effect. Four point difference was used to construct a grid solution model to explore the flow characteristics of annular gas after overflow, and the distribution laws of physical properties of acid gas, wellbore pressure, temperature and other parameters with well depth were discussed. The results show that the physical properties of acid gas change significantly with well depth: the more acid gas in the wellbore, the smaller the deviation factor, the greater the density and viscosity, and the parameter changes near the pseudo critical point are more than 40%; Compared with pure methane, the volume expansion of mixed fluid containing acid gas near the wellhead exceeds 20%, and the flow rate increases by more than 10%, resulting in increased well control risk. The study clarified the migration law of acid gas overflow in high temperature and high pressure gas wells, which has guiding significance for optimizing well control design, improving well control emergency plan, and formulating well killing measures.