The sustained annular pressure problem is common in ultra-deep gas wells, posing a serious threat to oil and gas production safety. Therefore, a gas flow and heat transfer model was established to obtain the temperature and pressure distribution of the wellbore. Then, using wellbore temperature and pressure as boundary conditions and considering real gas effects, a gas leakage rate calculation method was proposed. Based on the annular volume balance principle, a sustained casing pressure prediction model was developed. Finally, the reliability of the prediction model was verified using field test data from ultra-deep gas wells. The results show that the adiabatic coefficient is a key parameter in gas leakage rate calculations. Under real gas conditions, the adiabatic coefficient decreases with increasing temperature and increases with pressure. Compared to the current model, the traditional method based on ideal gas significantly underestimates the mass flow rate by about 22%. The predicted values agree well with the measured values. When the annular pressure exceeds the maximum allowable value, the pressure source should be diagnosed in time, and the leakage location and extent should be identified. The study provides theoretical guidance for annular pressure analysis and on-site safety management in ultra-deep gas wells.