The invention relates to a biological agent for preventing and treating ditylenchus destructors in Chinese angelica. The agent is prepared with the crude drugs by weight: 10-12g of mylabris, 12-15g of gemma agrimoninae and 8-10g of stone-like omphalia. The preparation method comprises the steps of: adding 55ml-60ml of concentrated hydrochloric acid into the above crude drugs, acidifying the mixture and leaving it to stand for 30min, conducting filtration, and reserving filtrate; adding 150ml of acetone into the filtrate to carry out ultrasonic countercurrent extraction for the first time for 45min, conducting filtration and reserving filtrate for standby application, adding 130ml of acetone in the generated filter residue to carry out ultrasonic countercurrent extraction for the second time for 45min, conducting filtration, mixing the obtained filtrate with the filtrate of the first extraction and placing them in a vacuum concentrator, reclaiming acetone, leaving the mixed filtrate for 10-15min and performing filtration to take precipitates; washing and refining the precipitates with 26-30ml of a mixed solution prepared by petroleum ether and absolute ethyl alcohol in a proportion of 1:1, thus obtaining the agent. The agent is diluted with hot water in a proportion of 90-120 microgram: 1g, and the medicinal material Chinese angelica, with the seedling root immersed by the diluted agent, can be planted.