This invention relates to a micro-ecological agent and the preparation method thereof, belonging to the micro-ecological agent technological field. The preparation of the active bacteria agent of this invention comprises: using L-calcii lactas and four useful micro-organisms of clostridium bytyricum, Lactobacillus casei, bacillus subtilis and saccharomycete as characteristic elements, cultured purely or mixedly by means of universal fermentator, producing active bacteria powder by freeze-drying, and acquiring the finished products by double crossing of the powder at a specific proportion. By using the anaerobic and aerobic characteristics of related bacterial strains, this agent has the following merits: playing the synergy effect in enhancing the disease resistance of marine animals and the water purification of aquaculture, substituting antibiotics fully or partly for the disease prevention and cure in fresh water aquiculture and mariculture, and improving the economic benefits of aquaculture. The L-calcii lactase component can accelerate the husking growth and prevent the disease of soft husk of shellfish marine animals like shrimp and crab. This agent is characterized of being nontoxic, free from side effects, nonpersistent, without secondary pollution and no drug tolerance, and will have a wide commercial application in the fresh water aquiculture mariculture.