Thin paper presents a series of experimental results of the trawl cod-end with cover--net for hairtail in Doug Hai and Huang Hai Seas. The experiment indicated that the Length of fish caught generally more than 50% of selected length (L_0.05)when the fish girth (the blocked position of the hairtail equal to internal circumfe-rents of the mesh. The difference between the internal circumference of the mesh caeulated from fish girth and the diameter of the mesh determined by the 50% of selected length were compared. For the purpose of protecting the resource of hairtail,the minimum internal diameter of cad-end mesh should be determined at 56mm. It is obtained from the interrelated formula concern with internal circumference of the mesh and the fish length. According to the selectivity of mesh and tho biological data of hairtail,the feasiblity of this mesh size was discussed.