The invention discloses a municipal sewage treatment agent and a preparation method thereof. The municipal sewage treatment agent is prepared from white alum, calcium aluminate, ferric trichloride, kaolin, polyacrylamide, polyferric sulfate, lime, sodium carbonate, ferric chloride, bentonite, plant fiber and coal ash; the municipal sewage treatment agent comprises the components: 30 to 50 parts of the white alum, 8 to 12 parts of the calcium aluminate, 10 to 20 parts of the ferric trichloride, 10 to 20 parts of the kaolin, 0.8 to 1.2 parts of the polyacrylamide, 1 to 1.4 parts of the polyferric sulfate, 8 to 12 parts of the lime, 4 to 6 parts of the sodium carbonate, 11 to 13 parts of the ferric chloride, 6 to 8 parts of the bentonite, 2 to 6 parts of the plant fiber and 10 to 20 parts of the coal ash. The municipal sewage treatment agent has the advantages of wide source of the production raw materials, environment friendliness and no toxin, good output water quality and high process operability. The medicament disclosed by the invention can be used for treating various types of sewage, such as chemical wastewater of oil fields, mines, steel, electroplating wastewater, dyestuff, pesticide, wine production and chemical industry, and also has a good treatment effect on domestic sewage, wastewater of paper mills, river and lake water, garbage seeping water, leather making wastewater and the like.