To explore the antifungal activity and potential mechanism of p-Hydroxybenzoic acid against Colletotrichum gloeosporioides in postharvest Citrus. The addition of p-HBA in-vitro and in-vivo was conducted to test the inhibitory effect. Dynamic changes of peroxidase (POD), polyphenoloxidase(PPO), Superoxide Dismutase (SOD), Ammonia-lyase (PAL)activity and the content of Malondialdehyde(MDA) with different treatment was studied in postharvest citrus. p-HBA displayed strong antifungal activity to C.gloeosporioides，when the concentration was higher, the effect was increased. The value of EC₅₀ is 0.056 mg/mL. The activities of PPO, POD, SOD and PAL were increased when inoculated with p-HBA and C.gloeosporioides, while the content of MDA was decreased. p-Hydroxybenzoic acid has a strong antimicrobial activity, which can induce the increase of defending enzyme’s activity and decrease the peroxidation of membrane lipid, The significance obviously is there for citrus fresh-keeping.