To study the characteristics and evolution of the gas-liquid two-phase flow pattern in the rod bundle channel of pressurized water reactor, based on the double-layer wire mesh sensor, the air-water two-phase flow pattern experiment of the 3×3 rod bundle channels was carried out at room temperature and pressure. The flow patterns include bubble flow, cap flow and slug flow. The experimental results show that, the critical bubble diameter range for the reversal of lateral lift direction under normal temperature and pressure is 4 to 5.8 mm. In addition, for bubbly flow, the time-averaged void fraction exhibits a “wall peak” distribution at lower superficial gas velocities and a “central peak” distribution at higher superficial gas velocities. For the cap flow, the cross distribution of cap shaped bubbles within adjacent subchannels triggers large-scale mixing of the liquid phase between adjacent subchannels, and the time-averaged void fraction exhibits a “central peak” distribution. For slug flow, large-sized bubbles develop along the axis and cross subchannel gaps to aggregate into slug shaped bubbles, with a more pronounced distribution of the central peak of void fraction. The experimental data are used to evaluate three drift-flux models. The Bestion’s drift-flux model overestimates the drift velocity, resulting in underestimated void fraction predictions. The Ozaki’s drift-flux model provides more accurate predictions of void fraction than the Xu Han model, with an average relative error of 9.8%.