In order to facilitate fish processing after harvest, automatic grading on its size is necessary, which is an important part of the pre processing. Based on a certain proportion of the dimensional characteristics of the fish body, analyzing the grading principle of the fish,we initially identified the space of fish grading device. According to dimensional characteristics of different fish body parameters, an automatically grading device in wihich space can be adjusted was designed, and the composition structure and working principle of grading device were described. On the basis of device structural function, experiments have been done to this grading device at different inclinations and speeds of grading rollers. The result shows that the device grading method is reliable, with the largest mixed level rate of 5.83%, and with a processing capacity of 2.1 t/ h, can meet the design requirements.