The invention discloses a synergistic agent for improving the antibacterial medicine effect of neomycinsulphate. Particularly, arginine and the neomycinsulphate are used in a combined way so as to enhance the medicine effect of the antibacterial medicine of neomycinsulphate, and the dosage of the neomycinsulphate is further reduced. The condition that the extermination or clearing effect on aeromonas hydrophila can be greatly improved through the combined use of the arginine and the neomycinsulphate is discovered in for first time; the treatment effect of the fish body aeromonas hydrophila can be improved through the injection or the oral administration of the two kinds of compositions; the fish body survival rate can be greatly improved. Therefore the arginine provided by the invention can be used as the synergistic agent of the neomycinsulphate; the arginine and the neomycinsulphate can be applied in a combined way for preparing composite antibacterial medicine; the medicine effect of the antibacterial medicine can be improved; the medicine consumption can also be greatly reduced; the toxic and side effects of the medicine and the medicine residue can be further reduced.