The invention relates to a harmless discharge treatment device of carbon dioxide, which comprises a seal box body, wherein the seal box body is internally equipped with a liquid dispersing device, a light-emitting device, a gas-liquid contacting plate and a magnetic field generator; the seal box body is divided into an upper box body and a lower box body; the middle of the seal box body is penetrated; the gas-liquid contacting plate is fixed in the upper box body; the liquid dispersing device is located above the gas-liquid contacting plate; and a feed port is formed at a connecting part of the liquid dispersing device and the upper box body. The device can effectively solve the problem of overproof discharge of carbon dioxide of a heat engine plant/thermal power plant, achieves culture and utilization of algae, is green and environmentally-friendly and has high economic benefits.