The invention relates to the technical field of traditional Chinese medicines, in particular to a traditional Chinese medicine composition for preventing and controlling bacterial diseases of aquatic animals and a preparation method and a plant antibiotic thereof. The traditional Chinese medicine composition includes Croton cascarilloides Raeusch roots and macleaya cordata. The Croton cascarilloides Raeusch roots are selected as a raw material and are supplemented by the macleaya cordata to prepare a novel traditional Chinese medicine composition having a remarkable bacteriostatic effect. It is found through study that the Croton cascarilloides Raeusch roots and the macleaya cordata are used in a compounded mode, the bacteriostatic effect is remarkably strengthened and is superior to existing products, and the traditional Chinese medicine composition can be widely applied to preparation of a bacteriostatic agent, especially a bacteriostatic agent for aquaculture.