A system of online weight detection and weight classification for freshwater fish was designed.The system detected the fish weight with an electronic weighing module.Weight signal of fish body was collected by the pressure sensors when the fish reached weighting area.The signal was sent to the control chip for storage and analysis after processed by the conditioning circuit.After figuring out the weight of the fish,the control chip judged the weight grade of the fish and sent the grade information to the fish distribution mechanism for distribution.The photoelectric sensors sent location information to the control chip after the fish reached the sensors installed on the fish conveyor.The controlling system allocated corresponding mechanisms to finish distribution depending on the weight grade and location information.Different weight sets were used to test the accuracy of the weighting device.The results showed that the weighting error was between ±0.73% when the weighting speed was between 0.16 m/s and 0.24 m/s.Results of performance test showed that the system can process 26 fish per minute with the classification accuracy of more than 98%.The weighing speed was 0.20 m/s.The distribution speed was between 0.20 m/s to 0.25 m/s.The fishes weighed between 300 g to 3 000 g were divided into 4 classes.