The net performance of tuna purse seine is influenced by mesh size, leadline weight, flow velocity and other factors. Most of the purse seine fishing vessels in China are used vessels. The age of the vessels is high and the equipment is old. The settling performance of the nets is hardly improved by increasing the weights of the leadline. In order to understand the effect of mesh size on the sinking performance, enclosed effectiveness and net shape, the authors based on our previous research results, net performance measurement at sea and the opinion of the captain, enlarged the mesh size from 260 mm to 390 mm (equivalent to 50%) of the low one third of the main body of the tuna purse seine currently used by the Shanghai Ocean Fishery Co., Ltd. Two model nets with different mesh sizes in the main body were made by the Tauti rule and the comparative tests were conducted through three shooting patterns at 8 cm/s and one at still water in a professional purse seine test tank of Nichimo Co., Ltd. of Shimonoseki Research Institute. The results show that:both the sinking depth and sinking speed of the experimental net with bigger mesh size (45 mm) are greater than that of the control net with smaller mesh size (30 mm)(Sinking depth increased by an average of 1.6%, and sinking speed increased by an average of 1.1%), indicating that increasing the mesh size can improve the sinking performance of purse seine. The pursing time of the experimental net was 14.3% less than that of the control net, and experimental net also kept a good form. The opening areas surrounded by the floatline at the end of shooting are almost identical for both the experimental net and control net; however, at the end of pursing, the opening area surrounded by floatline of experimental net was 1.33 times that of the control net and the opening area of the leadlline was 1.79 times that of the control net, indicating that the bigger mesh netting can provide bigger areas for fish school swimming.