The invention discloses application of liquiritin to prevention and treatment on crop oomycetes and fungal diseases. Specifically, liquiritin is used as an active component to make a plant fungicide together with pesticide acceptable auxiliary materials, or the liquiritin is directly used to prevent and treat crop oomycetes and fungal diseases. The liquiritin disclosed by the invention is naturalin source, free of toxic and side effect and safe to human beings and livestock, can be used as a low-toxicity effective antibacterial component, is not liable to cause drug resistance of pathogenic microorganisms, can be used for preventing and treating crop diseases such as tomato grey mold, cauliflower stalk break, pepper phytophthora blight and strawberry phytophthora blight caused by phytophthora capsici, phytophthora cryptogea, botryis cinerea, sclerotinia sclerotiorum and the like, and has great potential.