The invention relates to an ultraviolet flue gas treating device which comprises a mechanical filtering box body (1), an ultraviolet treating box (2), a water spraying tower (3), a dehumidifying tower (5), an electrostatic dust removing tower (6) and an activated carbon absorbing tower (7). After flue gas is treated, hazardous substances in the flue gas are completely and innocently treated; meanwhile, the electrostatic dust removing load is reduced, and the work efficiency is improved.