The invention relates to a biological preparation for controlling water quality deterioration and a preparation process thereof. The biological preparation is characterized by comprising the following components in part by weight: 40 to 60 parts of light rare earth, 15 to 35 parts of amino acid, 1 to 10 parts of ferrous sulphate, 0.5 to 5 parts of copper sulfate, 0.5 to 2 parts of zinc nitrate, 2to 8 parts of acid hydrolyzed vegetable protein, 1.5 to 3 parts of boric acid, 0.5 to 2 parts of sodium benzoate and 5 to 15 parts of water. The preparation process of the biological preparation comprises the following steps of: 1) testing raw materials; 2) performing set dissolution; 3) soaking the rare earth; 4) soaking auxiliary materials; 5) mixing the rare earth and the auxiliary materials; 6) blanking; 7) stirring; and 8) storing, testing and packaging. The rare earth serving as the raw material is adopted, so the biological preparation has the advantages of effectively controlling water pollution, providing a good living space for aquatic animals and plants and bringing appreciable economic benefit to culturists, along with simple preparation process.