The size grading during fish farming is beneficial for growth management and feed control, improving aquaculture efficiency. A guided live fish grading device was designed to solve the problem of high labor intensity and low efficiency in domestic live fish grading. The structural composition and working principle of the grading device were introduced, and the mechanical analysis and parameter design of the fish conveying mechanism were carried out. Based on the relationship between fish body mass and body shape parameters, the morphological data of experimental crucian were measured, and the power function fitting method was used to obtain the equation of the relationship between crucian body thicknessand body weight, which serves as the basis for setting the distance between grading channels.When grading, the spacing between the guide plates on both sides of the grading channel is adjusted, and the grading channel is set to meet the fish specification requirementssothat fish's body through the grading channel. Live fish grading performance experimentalwas carried out, with fish weight range of 0.14~0.33kg and thickness range of 30~39mm. Grading tests and effect analysis were conducted under three conditions of fish conveying speeds of 0.15 m/s, 0.20 m/s, and 0.25 m/s. The experimental results show that under normal conditions, the grading accuracy can reach over 90%, and the appropriate grading speed and grading channel setting have higher efficiency. This guided grading device can quickly and accurately grade live fish, improving the efficiency of mechanized operations in the breeding process.