A freshwater fish body mass online grading system based on virtual instrument was designed since the present freshwater fish body mass classification is slow and has low accuracy and poor reliability. The key parameters of the motion control module, information collection module and classification execution module were determined, and a method synchronously tracking freshwater fish body mass information and hierarchical pipeline was researched. The speed of the grading line was 150 mm/s, the correlation coefficient of the measurement results and the real body mass was equal to 0.999 8, the maximum relative error of freshwater fish body mass was 1.04%, the grading accuracy rate of the freshwater fish body mass grading system was 100%, and it handled 3 600 freshwater fish per hour. The test results show that the system can achieve fast, accurate detection of freshwater fish weight, and provide technical support for body mass online grading of the freshwater fish and other agricultural products with irregular shapes.