At present, freshwater fishing are mainly carried out by artificial capture, using ground cages, gill nets and trawls, which are high labor intensive and  low in efficiency, and can not meet the requirements of adjustable fishing in large open waters. By analyzing the living habits of fish in large lakes and reservoirs, this paper designed and made a set of fishing device on large water surface that is adjustable by changing the mesh size. The device is made of a small mesh river blocking net, the bottom is compacted and sunk by a net wrapped with small gravel, and then a fishing platform is built around by a pontoon to facilitate later fishing. The device can catch at any time regardless of slack or peak seasons, and has the advantages of simple manufacture, low cost, easy material sources and so on. Compared with the traditional "gill net + ground cage" fishing method, a fish capture experiment was carried out at the same time, and the three biological signs of fish were measured: total length, body length and body weight. The results show that when large-size fish (total length more than 10cm) fishing is required, the single catch amount of adjustable fishing device is 1.30 times (P = 0.002 < 0.01), the quantity 2.95 times and the survival 3.5 times, higher than the traditional method,. When small size (total length more than 1cm) fish fishing is required, the number, weight and species of fish by the adjustable fishing device are significantly greater than those by "gill net + ground cage", and the survival rate is also significantly improved. In conclusion, the fishing volume (quantity, weight and type) of adjustable fishing device in both selective fishing and all fishing is greater than that of ordinary nets, and the difference is significant. This device primarily  meets the requirements of adjustable fishing, and provides reference for the design of adjustable fishing device for large open waters.