The invention provides a polymyxin biopesticide agent for preventing and treating soil-borne disease for plants. The polymyxin biopesticide agent comprises the following raw materials by weight percentage: 5-30% of polymyxin, 5-12% of plant essential oil, 8-15% of a surfactant, 0.005-0.02% of citric acid, and the balance of water; the polymyxin is one of polymyxin B, plymyxin B sulfate, polymyxin E, and plymyxin E sulfate; and the plant essential oil is one or a mixture of garlic essential oil, folium eucalypti essential oil, mint essential oil, litsea cubeba essential oil, cumin essential oil, and lemon essential oil. The polymyxin and the plant essential oil are subjected to complex formulation to prepare the biopesticide agent, synergism of the polymyxin and the plant essential oil has good effect for preventing and treating a plurality of soil-borne diseases, and especially has obvious effect for preventing and treating tobacco black shank, cabbage clubroot, root rot of pseudo-ginseng, and root rot of ginseng, and the polymyxin biopesticide agent has the advantages of wide antimicrobial spectrum and high antibacterial efficiency.