NOVELTY - Method for preparing liquid fertilizer synergistic agent by performing genetically engineered bacterial fermentation on seaweed extract involves constructing genetically engineered bacteria, selecting fermented seaweed, extracting genomic DNA of lactic acid bacteria, selecting yeast cytoplasmic extract as vector, and combining to recombinant plasmid DNA, selecting Escherichia coli as host, culturing the constructed genetically engineered bacteria, fermenting cultured strain, and performing e.g. ion exchange demineralization, and vacuum evaporation.    USE - The method is useful for preparing liquid fertilizer synergistic agent (claimed).    ADVANTAGE - The method enables to prepare liquid fertilizer synergistic agent which enables to retain nutrition substance of the seaweed, delay urea decomposition and promote growth of crops.    DETAILED DESCRIPTION - Method for preparing liquid fertilizer synergistic agent by performing genetically engineered bacterial fermentation on seaweed extract involves constructing genetically engineered bacteria, selecting fermented seaweed capable of degrading macromolecules of photosynthetic bacteria, extracting genomic DNA of lactic acid bacteria as donor cells, selecting yeast cytoplasmic extract as vector, and connecting recombinant plasmid DNA, selecting Escherichia coli as host, where the recombinant plasmid DNA gene is introduced into E.coli cells, screening the constructed genetically engineered bacteria, and culturing the bacteria in culture medium for 12 hours, and screening for good strains, culturing and screening the strains for five generations by selecting and culturing the excellent strain in three-level seed culture for 8 hours, adding the cultured strain into 200 l seed tank culture for 10 hours and then adding the cultured strain into 1000 l seed tank culture for 12 hours, fermenting the cultured strain for 16 hours, and adding the supplementary material, continuously fermenting the mixture for 30 hours, filtering the solid impurities using microfilter and film filter, performing enzymolysis on the filtrate, filtering the mixture using ultra-filtration membrane, performing ion exchange demineralization process, and performing vacuum evaporation and concentration process.