Abstract: Liquid holdup is one of the basic parameters of gas-liquid two-phase flow. A method to detect the liquid holdup based on the gas-liquid two-phase flow video, which is captured using high-speed camera system in the horizontal tube, is proposed. Because the two-phase stratified flow image taken under natural light appeared dim between the gas-liquid region and interface of gas-liquid and difficult to directly extract their margin, this new method adopts new periodic changing rule to enhance the grayscale of the images to make them more clear, uses medium line calculation method to extract the gasliquid interface line and then calculates the liquid holdup using the extracted dada of distance between gas-liquid interface and the bottom of the pipe. The calculated liquid holdup includes the liquid holdup of different sections in the same image frame and the ones in the same section but at different time. Practical application shows that under the condition of severe natural light interference, the proposed method is able to obtain continuous, clear gas-liquid interface, and therefore can be applied in the analysis and detection of other flow pattern images.