Abstract: An experimental system was set up to study the gas-liquid two-phase flow in a vertical rectangular narrow channel, based on which, the gas-liquid two-phase flow was visualized by a high-speed camera, and subsequently the automatic images were recognized, while close bubbles were discriminated using Matlab software. Results show that the relationship between average void fraction α and volume void fraction β is found to be α=0.88β. The calculated results have a high matching degree with the experimental measurements after the coefficient C of Chen model is revised.