The problem of effective feeding for swimming fishes in recirculating aquaculture system (RAS) was studied. In order to solve this problem, a novel assessing method for the feeding activity of shoal was proposed with the help of tilapia which was used as the experimental objective, and the proposed method was based on the improved kinetic energy model. Instead of the complex segmentation of foreground of shoal and the tracking of individuals among shoals, the change characteristics of reflective areas of water surface caused by feeding activity of shoal were regarded directly as the key factor in this method. Firstly, the reflective areas of water surface were segmented and extracted in HSV-based color space. Then, with the help of the Lucas-Kanade optical flow, statistics method and entropy, the disorder degree of the change in reflective areas was calculated and analyzed. Finally, combined with information of the changing magnitude of reflection areas, the intensity assessment of feeding activity of shoal was executed. According to the comparison and analysis of the experimental results, the proposed method showed good description in intensity of feeding activity of shoal with different digesta index of stomach and bowels (20.35±10, 150.61±10) under strategy of over-satiation feeding through intermittent feeding at intervals of (40±2) s.