Abstract: Large vertical pipes are key structures connecting subsea wells to offshore platforms. However, existing studies mainly focus on small vertical pipes. In a vertical acrylic pipe with 80 mm inner diameter and 11 m height, a high-speed camera was used to visually research the influences of pipe diameters, liquid properties and inlet effect on air-water co-flow characteristic. Different flow regime maps of vertical pipes (diameters are in the range of 50–189 mm) were compared and the critical gas velocity of the transition boundary from bubble to slug flow tended to increase with the increase of diameters at D ≥ 80 mm. Drift-flux models were established in different flow regimes and liquid properties have a significant effect on drift coefficients of bubble flow and slug flow (void fraction α ≤ 0.4). The influence of inlet turbulent effect on the gas-liquid interface distribution gradually weakened and disappeared from the pipe base to 85D, where the flow was fully developed. Slug frequency has a trend of increase first and then decrease with the gas Weber numbers increasing at low liquid superficial velocities (JL ≤ 0.31 m/s). And on the basis of this law, a new slug frequency correlation was proposed. It was found that there was an exponential relationship between the ratio of lengths of Taylor bubble to slug and the void fraction.