The invention discloses a microbial agent for purifying sanitary sewage and a preparation method of the microbial agent. Active ingredients of the microbial agent is formed by mixing the following raw material bacteria including thiobacillus denitrificans, bacillus subtillis, bacillus licheniformis, bacillus polymyxa, lactic acid bacteria, nocardia amarae, denitrifying bacteria, rhodopseudomonas palustris, thiophilic red pseudomonas, rhodospirillum, saccharomycetes and propionibacterium. By means of the microbial agent, cellulose, saccharides, fat protein, pathogenic bacteria, viruses and parasitic ovum in sanitary sewage and pollutants, such as inorganic salt which is high in the contents of nitrogen, sulphur and phosphorus, the BOD and COD removal effect is guaranteed, meanwhile good bactericidal ability is achieved, high inhibiting ability is achieved especially for pathogenic bacteria, and the effect of treating sanitary sewage is remarkable.