Abstract: Bubble motion in confined channels has important applications in such fields as carbon dioxide landfill and oil displacement, cardio-cerebrovascular and pulmonary gas-liquid transport. Key issues such as liquid film dynamics and the morphology evolution of the gas-liquid interface have drawn much attention. The purpose of this work is to explore the relative thickness distribution of the liquid film between gas and solid wall, by an experimental method based on light interference, during the motion of bubbles in a confined channel. In particular, we aim to identify the characteristics of liquid film thickness caused by periodically varying speed of bubbles. In the experiments, steadily moving bubbles and bubbles with accelerations were formed in a snake-shaped long channel and a straight short channel, respectively, by the flow-focusing method of droplet microfluidics. The relative optical interference intensity (ROII) method was used to obtain the thickness distribution of the liquid film by analyzing the fringes, which are formed by the interference between two beames of reflected lights at the gas-liquid and the liquid-solid interfaces, respectively. Our findings indicate that the liquid film caused by bubbles moving at a constant velocity shows a steady thickness distribution in the bubble’s moving reference frame, and a saddle-shaped thickness distribution at the rear of the bubble is obtained, which proves the validity of the experimental method. On the contrary, when the bubble undergoes periodic deceleration - acceleration - constant speed motion, the thickness distribution of the liquid film in the bubble’s moving reference frame changes periodically with time. The varying speed of bubbles causes the overall temporal evolution of the gas-liquid interface morphology and shows a hysteresis effect. This study of the liquid film thickness at varying speed of bubbles may provide phenomena and experimental data for theoretical and computational study on the bubble dynamics in confined geometries.