Abstract: Because the echo attenuation of ultrasonic on different medium interface is different, the method of non-intrusive on-line identifying of gas-liquid two-phase flow pattern by ultrasonic is proposed. Reflection coefficient of sound pressure when the medium contacted with the pipe inner surface is water and air is calculated, which proves the feasibility of identify flow pattern with ultrasonic. Numerical simulation is made to research the characteristics of echo attenuation. When the fluid in the pipe is water or air respectively, the implementation mode of identification method for ultrasonic flow pattern at different surface is obtained.According to the results of numerical simulation, on-line identification experiment on ultrasonic gas-liquid two-phase flow pattern is carried out, to analyze the distribution of gas and liquid at intersecting surface, and identify the flow pattern of gas-liquid two-phase flow in horizontal pipe. The results show that under the premise of pipeline running, the flow pattern can be acquired in real time accurately by ultrasonic flow pattern identification method in gas- liquid two-phase flow process, with easy operation and reliable result.