Sustained annular pressure seriously affects the safety of wellbores. However, most models take the annulus liquid thermodynamic parameters and tubing pressure as constant values. Therefore, the prediction model of wellbore temperature and pressure field was first established based on the basic theory of heat transfer to provide basic parameters for the follow-up sustained annular pressure calculation. Second, based on fluid mechanics theory, a prediction model of sustained annular pressure was developed considering the influence of tubing pressure fluctuation and thermostatic coupling on the thermodynamic parameters of annulus fluid. Then, the iterative method was used to solve the annular pressure. Combined with the field monitoring data of a high-pressure gas well, the model’s accuracy was verified. Finally, a parameter sensitivity analysis was performed, and annular pressure regulation steps were determined. The obtained results revealed that the developed prediction model met engineering accuracy requirements. In a high-pressure gas well, after repeated cycles of shutting in and opening the well for 19 times, gas column height was equal to the leakage point position. Gas column height, leakage point depth, shut-in well annulus temperature, and liquid density were found to affect the change speed and stable value of the annular pressures of shut-in and open wells. The sensitivities of leakage point depth and shut-in annulus temperature to annular pressure regulation were the highest and lowest, respectively. Research results helped us reasonably and quickly manage and control sustained annular pressure gas wells.