The invention discloses a preparation method of a decatungstate composite iron oxide photocatalyst. The preparation method comprises following steps: at 40 to 50 DEG C water bath stirring conditions,a sodium carbonate solution with a concentration of 0.5 to 1.0mol/L is added into a iron chloride solution with a concentration of 0.5 to 1.0ml/L drop by drop, wherein the Na<+>/Fe<3+> is controlled to be 0.9 to 1.1; after dropwise adding, an obtained product is subjected to aging for 24 to 36h in water bath continuously so as to obtain a polymerization hydroxyl-iron solution; a Na2WO4.2H2O solution with a concentration of 20 to 40mmol/L is added into a hydrochloric acid solution with a concentration of 1 to 3mol/L, and the molar ratio of Na2WO4.2H2O to hydrochloric acid is controlled to be 1:2, the pH value is adjusted with diluted hydrochloric acid to 1 to 2, heating is carried out for 30 to 60min at 85 DEG C in water bath so as to obtain decatungstic acid; the polymerization hydroxyl-iron solution is added into the decatungstic acid solution drop by drop, stirring is carried out continuously, water bath temperature is maintained to be 85 DEG C, reaction is carried out for 40 to 60min, filtering, washing, and drying are carried out, and an obtained product is subjected to sintering for 50 to 100min at 400 to 550 DEG C so as to obtain the decatungstate composite iron oxide photocatalyst. The efficiency is high; no pollution is caused; and a plurality of times of recycling can be realized.