Accurate prediction of annular pressure distribution is considered fundamental for the design of oil-casing annular gas-lift systems. Slug flow is recognized as the most common flow regime in gas-liquid two-phase flow within wellbores. In high gas-liquid ratio annular multiphase flow, experimental studies remain limited, and the accuracy of pressure drop predictions is insufficient. Water and air were employed as experimental fluids, and two annular configurations with dimensions of 38.5 mm×62 mm and 38.5 mm×76 mm were selected for experiments conducted in an 11.5 m long experimental pipeline. The experimental results indicate that the flow characteristics of slug flow in the annulus differ significantly from those observed in circular pipes. In annular slug flow, Taylor bubbles are no longer symmetrically distributed, and a liquid flow channel is formed between the Taylor bubbles and the inner pipe wall. Based on the slug flow characteristics obtained from experiments and numerical simulations, and by integrating existing theories for circular pipes and annular gas-liquid flow, a pressure drop model for slug flow is established, taking into account the effects of annular geometry and inclination variations. Validation against experimental data shows that the proposed slug flow pressure drop model predicts the pressure drop with an average error of approximately 15%, which satisfies the requirements for engineering applications.