In order to accurately and efficiently evaluate the number of fish in the cage and scientifically guide the cage culture, a method of counting the number of fish in the cage based on forward-looking sonar was proposed. Methods In the application of the method, based on sonar image processing technology, the acoustic image threshold of a single fish body was divided based on the acoustic calibration results of the fish cultured in the target cage in the  anechoic pool laboratory. Secondly, controling sonar rotation according to the PTZ, the traversal detection of the fence area is carried out. Then, the threshold value determined by the acoustic calibration results was used as the basis for classification, and the fish quantity in the whole aquaculture cage was estimated by dividing and counting the fish in the acoustic images measured from different angles.. Finally, combined with the detection Angle and location information, the spatial distribution characteristics of fish in the cage culture area were obtained, which could be used to guide fishermen to carry out follow-up cage culture. The method was applied in the cage culture of large yellow croaker in Zhoushan Qingbang Island, and the results showed that the estimated quantity of large yellow croaker in the cage was basically consistent with the artificial evaluation results. After several calculations, the average relative error is only 2.45%. The realization process is more efficient and convenient, and it has a wide application prospect in the subsequent cage culture guidance. It can effectively provide fish quantity information for aquaculture cages, and provide scientific and reliable quantity statistics methods for the cage culture industry, which is in urgent need of fish quantity supervision, and has a wide range of application scenarios.