The invention discloses a biology treating agent for inhibiting water eutrophication and a preparing method thereof. The biology treating agent is prepared from, by mass, 8-15 parts of radix mirabilis, 5-10 parts of clove, 3-7 parts of Xanthium sibiricum, 30-45 parts of modified diatomaceous earth, 40-50 parts of poly aluminum ferrous chloride, 12-15 parts of ferric sulfate, 12-15 parts of trichloro-isocyanuric acid, 20-25 parts of potassium citrate and 25-30 parts of disodium edetate dihydrate. By means of addition of the radix mirabilis, the clove and the Xanthium sibiricum and calcination treatment, the adsorption property of the modified diatomaceous earth and effects such as coagulation and electrical neutralization of polycation are effectively brought into play, alumen ustum is large and compact, the sedimentation speed is high, the effects of removing suspended substances like blue-green algae, turbidity, nitrogen and phosphorus and the like are remarkable, treated water is transparent, the water quality and the acid-base property are stable, and the biology treating agent has effects of many kinds of agentia such as a corrosion inhibitor and bactericide, so that water eutrophication is prevented.