The invention discloses a biological enzyme purifying agent for treating black and odorous river courses. The parts by weight of each component are: 20-50 parts of biological enzyme, 10-15 parts of Rhodococcus erythropolis sludge flocculant, chitosan quaternary ammonium 10-18 parts of salt, 10-15 parts of grape seed extract, 5-15 parts of sweet potato extract, 10-15 parts of polyaluminum chloride, and 8-12 parts of polyacrylamide. Under the joint action of multiple components, the present invention has remarkable water purification effect, can significantly improve water quality, increase dissolved oxygen content in water, inhibit the growth of harmful bacteria, remove suspended solids and harmful algae in water, and reduce ammonium nitrogen and nitrate levels in water. Nitrogen content, clarify water body, eliminate odor, promote the degradation of pollutants, and promote the healthy development of water ecology. The present invention has abundant raw material resources, simple preparation process of the water purifying agent, short production cycle, simple operation, low production cost, and is suitable for industrialized production.