Experiments showed that the 50 % retention length (L0.5)increased with the meshsize of cod-end .When the mesh-size of cod-end increased from 40mm to 50mm with 5mm intervals , 50 % retention lengths (L0 .5)were 85 .88 , 90 .26 , 95 .32mm for Parapenaeopsis hardwickii , 87 .58 , 88 .09 , 92 .56mm for Solenocera crassicornis and 85 .04 , 85 .96 , 88 .32mm for Trachypenaeus curvirostris respectively .The relations between the mesh-size of the cod-end and 50 % retention length of the three species mentioned above can be described as follows : P .hardwickii L0.5 =48 .03 +0 .940Me , S .crassicornis L0.5 =67 .00 0 .498Me , T .curvirostris L0.5 =71 .68 0 .328Me , Mixed up three species : L0.5 =60 .59 0 .636Me , It is necessary to increase the mesh-size of cod-end from the range of 20mm～ 30mm to the range of 35 ～ 40mm in fishing operation .As at the latter range of mesh-size , 50 %retention length (L0.5)for P .hardwickii , S .crassicornis and T .curvirostris were 80.93 ～ 85.63mm、84.43～ 86 .92mm and 83 .16 ～ 84.8mm respectively , and 82 .85 ～ 86 .03mm If the three main shrimp species were mixed up .It will help to release juvenile shrimps and fish and in the meanwhile improve the size and quality of shrimp catch .