The invention discloses a compound biological bactericide special for trees and a preparing method, and belongs to the technical field of biological bactericides. The compound biological bactericide is composed of a compound effective microbial community A and a compound effective microbial community B. Expanding propagation and culture are conducted on trains in a seed tank; after culture is conducted for 24 hours in the seed tank at the culture temperature of 32 DEG C, the strains are transferred into a fermentation tank for fermentation culture; culture is conducted for 50-72 hours at the culture temperature of 30-37 DEG C; when the strain formation rate reaches 80% or above, fermentation ends, and a bacteria solution is obtained. The bacteria solution promotes the flourishing of plantroot systems and the vigorous growth of branches and leaves; through the natural anti-bacterial property and anti-disease property of beneficial microorganisms, plants are protected against bacteria invasion, the stress resistance of the plants is enhanced, and diseases and insect pests are reduced. Thus, the consumption and residues of chemical fertilizers and pesticides can be reduced, and the compound biological bactericide can effectively repair the environment, improve soil and increase the yield and quality.