The invention relates to a water-resistant sulfur-resistant flue gas denitrification powder catalyst, a preparation method and an application of the flue gas denitrification powder catalyst. The catalyst comprises main components including titanium dioxide, manganese oxide and auxiliaries, wherein the auxiliaries are two of or a combination of more than two of zirconium oxide, copper oxide, molybdenum oxide, cerium oxide, iron oxide, nickel oxide, cobalt oxide, niobium oxide and tungsten oxide; the mole ratio of Mn to Ti is 0.01-1.0, and the mole ratio of metal in the auxiliaries to titanium metal elements is 0.01-1.0. The catalyst is covered with one silicon dioxide coating or more silicon dioxide coatings. The catalyst is prepared with a coprecipitation method, can be used for an NH3-SCR (selective catalytic reduction) purification process of nitric oxide in flue gas in a fixed source or a moving source, adopts non-toxic and harmless raw materials and has the advantages that the preparation method is simple and convenient, the catalytic activity is high, the active window is wide, the water-resistant and sulfur-resistant stability is good, environment protection is realized and the like.