The invention provides a seed treating agent, and belongs to the technical field of pesticides. Active ingredients of the treating agent are thiamethoxam and brevibacterium, particularly 1 gram of preparation contains 2 to 200 billion brevibacterium, and the thiamethoxam accounts for 1 to 40 weight percent of the treating agent. The thiamethoxam and the brevibacterium are synergized with each other, and can provide global protection for seeds by acting on two different target points at the same time. The seed treating agent has good environmental compatibility, low toxicity and biosafety for human, animals, natural enemies and the like, and belongs to a broad-spectrum and high-efficiency biological seed treating agent. A dry-blending agent or suspension seed dressing agent can be prepared by adding proper auxiliary materials into the treating agent, can be prevent and control grub, wireworm, mole cricket and other underground pests in crop fields of corn, wheat, paddy and the like by seed dressing or seed pelleting, can prevent and control aphid at seedling stage, laodelphax striatellus, thrips palmi karny, trialeurodes vaporariorum, leafhopper and other insects with sucking mouth part, and can prevent and control blast, banded sclerotial blight, full rot, rice blast, botrytis, powdery mildew, bacterial wilt and other diseases.