The invention relates to a photosynthetic bacterium strain for preventing and treating clubroot of cruciferous crops and its application and belongs to the technical field of plant protection. The photosynthetic bacterium belongs Lampropedia sp, is named as photosynthetic bacterium GH-1, is preserved in the China center for type culture collection on November 25, 2016 and has a preservation number of CCTCC NO: M 2016680. The strain has high ability to effectively prevent and treat clubroot of cruciferous crops, can be used for preventing and treating clubroot of cruciferous crops, reduces clubroot morbidity to about 10%, observably reduces a plant disease index, effectively improves a crop yield, has a yield increasing rate of 150% or more, is environmentally friendly and safe, does not produce residues and broadens the application field of photosynthetic bacteria.