Gas-liquid phase detection is an important technique applied in a wide range of industries. In this study, we developed a phase detection method using a film-based optical waveguide. The optical waveguide is a thin and flexible film with multi-light paths that uses multi-microsensors for gas-liquid phase detection. The intensity of the reflected light generated by different refractive indices between gas and liquid aids in distinguishing the phase. Additionally, the sensing principle is identical to that of the typical optical fiber probing technique. In this study, we investigated the detection process considering the impact of a single droplet on waveguide sensors. Furthermore, we analyzed a droplet evaporation phenomenon and a thin-film liquid flow accompanied by a high-speed airflow on the sensors. Based on the obtained results, we determined that the proposed method can effectively measure the simultaneous local multipoint and high temporal resolution phase detection on a smooth surface. Therefore, we believe that our original sensor can diagnose such a dispersed two-phase flow near the wall inside of machines or curved tubes where the high-speed visualization is hard to be applied.