Two-phase Flow is widely involved in reactor design and is directly relevant to reactor safety. However, the flow regime in narrow rectangular channels still needs further study because it has a considerable difference from tube and bundle channels. To investigate the two-phase flow regime and interfacial structure characteristics, the air-water experiment with an adiabatic vertical channel of 4 × 66 × 1800, 6 × 66 × 1800 mm have been conducted under atmosphere pressure condition. The impedance void meter was used to measure the global void fraction in narrow rectangular channels. A high-speed camera was used to record the profiles of the flow regime. The flow regime was identified by the random forest clustering algorithm based on a training sample. The profiles of different parameters, including void fraction, pressure loss at Z/D = 150, were analyzed in this paper. Furthermore, based on the parameters’ distribution, the regime transition criteria in narrow rectangular channels were discussed. It is shown that the transition from bubble to slug flow always occurred when the average void fraction is 0.17–0.2. The transition value is 0.57–0.62 when the slug Flow changes to the churn-turbulent Flow and 0.78–0.8 from churn-turbulent to annular Flow. The constant used in the Lockhart-Martinelli correlation is found to calculate the frictional pressure drop in a rectangular channel. Furthermore, the drift-model applied to the rectangular channel is verified.