The invention discloses an application of a small-molecule compound produced by bacteria as a synergist of a chemical pesticide bactericide. The synergist is 3-oxydodecanoyl homoserine lactone (AHL) generated by pseudomonas aeruginosa (PA), and can be extracted from a fermentation broth of the pseudomonas aeruginosa or can be synthesized by artificial chemistry. A micro amount of the small-molecule compound (AHL) is mixed with the chemical pesticide bactericide for use, and has an obvious synergistic effect on prevention and control of the diseases caused by Erwinia chrysanthemi pv.Zeae. The compound disclosed by the invention can significantly improve a bactericidal effect of the chemical pesticide bactericide, greatly reduce a use amount of chemical pesticides and residues of the pesticides, can reduce costs, protect the environment, effectively alleviate the increasingly-serious problem of drug resistance of pathogenic bacteria, and promote achievement of a ''pesticide reduction'' indicator and green development of agriculture.