In order to solve the problem of high damage and low efficiency in the grading process of live fish, an improved scheme of rotary live fish grading device was put forward. Replacement of the way of CAM mechanism and spring return mechanism to rotate graded baffle with the way of pinion and rack mechanism to raise graded gate could reduce the manufacturing cost and fault incidence. The 3d model was established and the method of motion simulation of key components was given. First, based on the contour parameters and grading capacity of the target fish, the structure of the live fish grader and the key parameters of the graded gate were determined, the opening and closing conditions of the graded gate were analyzed based on the thickness parameters of the graded grass carp, and the design parameters of the grader were determined. Second, the kinematics model of the grader key mechanism in the rotary process was established, the contour line of the graded regulation track was designed, and the movement analysis of the liftgate was carried out. Finally, Solidworks was used to establish the virtual prototype of grader, the virtual prototype was imported into ADAMS virtual software, and relevant parameters were set for simulation analysis. The simulation results show that when the grading device operates at a motor speed of 12 r/min, the graded gate can complete three rise actions (each rise is 10 mm) and one return action, and the time of each rise or return can be controlled within 1.25s, indicating that replacing the original mechanism with a pinion and rack mechanism can also meet the movement requirements. The results show that the rotary live fish grader has stable maneuverability and can achieve smooth movement, which meets the requirements of live fish grading. The design of the device is reasonable, which provides the basis for the design of the test prototype.