The invention discloses the application of diclofenac sodium in preparation of a fungicide used for controlling plant diseases caused by plant pathogens. Through indoor toxicity measurement, it is proved that diclofenac sodium has good inhibitory activity against the plant pathogens. Diclofenac sodium is used as the fungicide for the first time, has the advantages of low toxicity and high efficiency, and is suitable for chemical control of plant diseases. Traditional chemical fungicides have high pollution to the environment and high residues and pose a serious threat to food safety. Diclofenac sodium is an environmentally friendly small molecule compound and is degradable and free of pollution, the drug resistance of plant pathogenic fungi is low, and the diclofenac sodium is safe to non-target organisms, human beings and livestock. The high quality of agricultural products, fruits and vegetables can be ensured, the requirements of sustainable development are met, and the diclofenac sodium has wide research and market application prospects.