The invention relates to a three-level sorting device for fishes based on weights and a working method. The three-level sorting device comprises a feeding mechanism, a conveying mechanism, a sorting mechanism and a control system. The feeding mechanism is located on one side of the conveying mechanism, the fishes are conveyed to the conveying mechanism, the sorting mechanism is installed on the conveying mechanism, the fishes are sorted according to the weights, and the feeding mechanism, the conveying mechanism and the sorting mechanism are connected with the control system and work after receiving an instruction sent by the control system. During work, the control system drives the sorting mechanism to act according to different weights, so that the fishes are sorted. By the utilization of the three-level sorting device for the fishes based on the weights and the working method thereof, the fishes can be sorted by three levels according to the weights in a full-automatic mode, the labor intensity is lowered for workers for sorting, working efficiency is improved, and the device can be widely applied to sorting the fishes.