The invention discloses a synergistic agent and a preparation method thereof, wherein the synergistic agent comprises the following raw materials according to proportion by weight: 70.8-87.5 parts of stone potassium, 3-8 parts of polyaspartic acid, 3-8 parts of maleic anhydride, 0.3-0.5 parts of azone, 5-10 parts of urea or ammonium hydrogen carbonate, 0.4-0.7 parts of d bacteria liquid, 0.3-0.8 parts of EDTA (Ethylene Diamine Tetraacetic Acid) and 0.2-0.5 parts of n-butyl alcohol. The preparation method of the stone potassium comprises the following steps: respectively crashing 50 parts of zeolite ore, 20 parts of phosphorite and 30 parts of leopoldite into 100-200 meshes and then uniformly mixing so as to obtain a mixture; taking 73.5-87.3 parts of mixture, 2-4 parts of urea, 2-5 parts of phosphorus pentoxide, 1-2 parts of potassium sulfate, 0.2-0.5 parts of ammonium molybdate or sodium molybdate, 1-2 parts of manganous sulfate, 3-6 parts of ferrous sulfate, 1-2 parts of zinc sulfate, 1-2 parts of sodium borate, 0.5-1 parts of copper sulfate and 1-2 parts of fulvic acid; and stirring and sealing up for keeping standby. The preparation method of the bacteria liquid comprises the following steps: uniformly mixing colon bacillus, Fusarium moniliforme, Corynebacterium pekinense, Streptomyces jingyangensis, Bacillus megatherium, Bacillus mucilaginosus and Streptomyces according to a proportion of 0.5%: 0.5%: 0.5%: 0.5%: 0.5%: 0.5%: 0.5%. The synergistic agent is multipurpose, nutritious, low in cost, non-toxic, nuisanceless and free of residue.