The invention belongs to the technical field of flue gas purification, and particularly relates to a flue gas integration removal system and method by means of gas-like phase preoxidation combining with absorption. The system comprises a gas-like phase reagent generating device, a preoxidation device and an absorption tower. The removal method comprises the following steps that 1, a gas-like phase composite oxidant is prepared; 2, a preoxidation reaction is performed, wherein the gas-like phase composite oxidant is utilized to efficiently oxidize NO and Hg<0> in a gas phase; 3, an absorption reaction is performed, wherein mixed liquor of sodium humate and ammonia water serves as absorption seriflux to absorb and remove SOx, NOx and Hg<2+> in oxidation products. According to the flue gas integration removal system and method by means of gas-like phase preoxidation combining with absorption, the two-level arrangement mode of combining preoxidation with absorption is adopted to perform integration removal on multiple pollutants of coal-fired flue gas, and therefore the problems that in an existing hierarchical arrangement mode, a system is complex, the floor space is large, and the operating cost is slightly high are solved; the desulfuration, denitration and mercury removal efficiency can meet the current pollutant discharge standard, a removal product is sulphureous and nitrogenous compound fertilizer, and higher economic and environmental benefits are achieved.