An image analysis software for entrained air concentration is developed based on object-oriented advanced programming technique VC in association with high speed digital camera. A digital image processing method which automatically recognizes the bubble in water-air two-phase flow is put forward, and an algorithm for calculating the two-dimension and instantaneous air concentration is presented. The study is a multidisciplinary application combining image processing technology with water-air two-phase measurement. The research results will be helpful to understand the mechanism of air entrainment in high speed flow, and will lay a good foundation for studying the application of image processing technology in the measurement of flow field, in particular, high speed water-air two-phase flow field. It’s also of great academic significance and practical value for improving the measurement of air concentration and its precision.