In order to solve the inaccuracy in the calculation of the wellbore pressure distribution caused by large liquid production in the Donghai gas field, the gas–liquid production conditions of the Donghai gas field were simulated with indoor experiments, and the flow patterns for different pipe diameters, different inclinations, and different flow patterns were systematically analyzed using a flow pattern discrimination method, liquid holdup calculation method, and pressure drop calculation method. Using the experimental data, the division methods for different flow patterns were screened. Finally, based on the fact that the change trend for the flow patterns was consistent, the Kaya–Sarica–Brill method was selected to establish the flow pattern discrimination formula. According to the calculation method for the Mukherjee and Brill (M–B) liquid holdup, the M–B model was re-established according to a 75 mm pipe diameter and 60 mm pipe diameter using the instantaneous liquid holdup measured in the laboratory. Through the comparison and analysis of the measured data and the calculated data for the Beggs and Brill (B–B_ pressure drop model under the same working conditions, it was found that when the B–B model was applied to different angles and different gas–liquid ratios, the error decreased with an increase in the angle and increased with a decrease in the gas–liquid ratio. After verifying the correlation of different dimensionless numbers that can characterize the gas–liquid ratio, it was considered that the introduction of the Reynolds number into the original model could greatly improve the accuracy of the calculation, so a new pressure drop calculation model was established. The new pressure drop calculation model takes into account the two parameters of the well deviation angle and gas–liquid ratio. The accuracy was greatly improved, as verified by field measurements in four wells.