In order to facilitate the harvesting and grading operations of the farmed fish in the shallow sea water enclosure facilities, floating-rope enclosure culture system was selected as the research object in this paper. According to the structure, size, the laying techniques and the key performance of the enclosure culture facility, combining with culture method and the production characteristics, a grading harvesting cage was designed and fabricated. The main body of the cage included steel bar frame, netting, ropes, grading gate and other materials. The cage body was cylindrical, and the top of the cage was inverted cone shape. Thetotal height was 1515mm, and the volume was about 1m3. The structure was divided into 3 parts: the fish entrance, grading gate and harvesting part. The results of fish grading test in the lab were as follows: when grading grid space was 15 mm, the averaged residual rate of small-size fish was 7.78% and the averaged escape rate of large-size fish was 2.38%; when grading grid space was 20 mm, the averaged residual rate of small-size fish was 4.75% and the averaged escape rate of large-size fish was 3.00 %. The results of the trial in the sea were as follows: the harvesting rate of the large-size fish was more than 90%. The study showed that through the acclimatization of daily feeding, fish could be attracted to enter  the cage, then the large-size fish could be harvested to the live fish tanks after lifting cages and driving the small fish inside to escape from the grading gate, so as to achieve the aim of harvesting large-size fish and leave the small-size fish.