The invention discloses an in-situ treatment technology for a black and odorous water body. A controlled release bacteria product and artificial seaweeds are simultaneously fed to a to-be-treated black and odorous water body at a set ratio, wherein the controlled release bacteria product is a solid product with a porous structure and is prepared from the following components in parts by weight: 25-45 parts of volcanic ashes, 20-30 parts of flyash, 10-20 parts of cellulose, 0.5-1 part of lauryl sodium sulfate, 5-15 parts of gypsum and 15-30 parts of composite bacteria liquid, wherein the composite bacteria liquid comprises the raw materials by viable counts: (1-2)*10<9>CFU / mL of photosynthetic bacteria, (3-4)*10<9>CFU / mL of nitrifying bacteria, (2-3)*10<9>CFU / mL of denitrifying bacteria, (4-6)*10<9>CFU / mL of ammonifiers and (5-10)*10<9>CFU / mL of phosphorus accumulating bacteria. For treatment of the black and odorous water body, the homemade controlled release bacteria product is selected to be combined with the artificial seaweeds after soaking the bacteria liquid, so that the in-situ treatment technology is high in treatment efficiency, small in occupied land, low in investment cost, free of energy consumption, free of operation cost, low in sludge quantity and high in anti-shock loading capability, and a system is stable and long in service life.