The invention belongs to the field of water pollution treatment, in particular to a water body algae scavenger and a preparation method thereof. A blue algae scavenger taking light mineral as a carrier is characterized in that the blue algae scavenger is prepared by a lanthanum nitrate solution, a sodium hydroxide solution and lumpish porous light mineral raw materials, wherein the addition amount of the sodium hydroxide solution is 1 to 5 percent of the volume of the lanthanum nitrate solution, and the addition amount of the lumpish porous light mineral is 5 to 10 percent of the total mass of the lanthanum nitrate solution and the sodium hydroxide solution; the mass concentration of the lanthanum nitrate solution is between 0.2 and 0.5 percent, and the mass concentration of the sodium hydroxide solution is between 5 and 10 percent; the diameter of the lumpish porous light mineral is between 0.1 and 3 centimeters; and the lumpish porous light mineral is one type or mixture of more than two types among floatstone, perlite and expanded perlite, and the proportion is random when more than two types are mixed. The blue algae scavenger prepared by the method has the advantages of good blue algae removal effect, quickness in taking effect, low cost and no secondary pollution.