The invention discloses a photosynthetic bacterial strain, a biocontrol bacterial agent, a biocontrol fermentation liquid, a preparation method and an application. The photosynthetic bacteria strain is Rhodobacter capsulata A18. (1) The production process of the photosynthetic bacterium Rhodobacter capsula A18 of the present invention is simple, the culture time is short, and the cost is low, and the bacterial strain can be prepared by a simple, fast, and low-cost operation method to obtain the biocontrol method for the prevention and control of citrus canker. Bacteria and biological control fermentation liquid. (2) The biocontrol agent and the biocontrol fermented liquid prepared by the photosynthetic bacterium Capsule Rhodobacter A18 of the present invention have the characteristics of environmental friendliness, nontoxicity to humans and animals, no phytotoxicity to crops, simple and convenient application, and are not easy to cause damage by diseases and insect pests. develop drug resistance.