The invention discloses a high-stability sewage treating agent and a preparation method thereof. The sewage treating agent comprises following materials by weight: 10-15 parts of citric acid, 50-70 parts of sodium carbonate, 10-15 parts of magnesium carbonate, 8-20 parts of ferric trichloride, 7-20 parts of calcium oxide and 10-15 parts of polyacrylamide. The preparation method includes uniformly mixing the materials in sequence and stirring. Compared with the prior art, the sewage treating agent and the preparation method has characteristics of multifunction, high efficiency, environment protection, high sewage treating speed, short treating time, capability of efficiently removing various pollutants and toxic materials with broad spectrum, high adaptability to high dirty concentration, wide pH value range and wide temperature change range of sewage, capability of effectively reducing odour of various types of sewage, high degree of alkalization, low corrosion, convenience for production, no pollution in production processes, simple product package, convenience for transportation, and low comprehensive cost.