The invention discloses a plant-derived monomeric compound, i.e., flavaspidic acid AB for effectively treating and preventing porcine reproductive and respiratory syndrome and resisting tumors. As proved by an in-vitro antitumor experiment, the flavaspidic acid AB has remarkable inhibitory actions on a plurality of malignant tumors such as cervical cancer, breast cancer, gastric cancer, lung cancer and liver cancer; and as proved by an in-vitro antivirus experimental result, the flavaspidic acid AB (20 and 40 mug/ml) can be used for effectively inhibiting porcine reproductive and respiratory syndrome viruses 0 hour, 1 hour, 2 hours and 3 hours after cell exposure, can act at the early stage of porcine reproductive and respiratory syndrome virus infection, and has the effect of inhibiting duplication or budding after virus uncoating. The flavaspidic acid AB can be taken as a medicament or a lead compound for preventing and treating tumors and porcine reproductive and respiratory syndrome infection.