The efficiency of heat and mass transfer during the process of the horizontal tube falling film flow is significantly influenced by the flow pattern of the fluid between the horizontal tubes, so it is important to pay attention to this factor in order to maximize the performance of the horizontal tube falling film evaporator. In this study, a horizontal tube falling film flow experiment platform is built in order to observe the flow patterns between horizontal tubes. Study and analysis of the mechanism of flow pattern transition is conducted. The "critical separation length" is the distance sign that there are droplets generating. In other words, column flow occurs when the tube spacing is below the "critical separation length". The drop-column flow is created as a result of the flow instability. The two crucial steps of the column-sheet transition process are accumulation and integration.