The invention discloses a device and a method for increasing grain size of PM2.5 through turbulent flow and chemical agglomeration coupling. Dust-containing flue gas sequentially flows through a chemical agglomeration area and a turbulent flow generation area in a flue and finally enters an electrostatic dust collector; when the dust-containing flue gas flows through the chemical agglomeration area, the PM2.5 particles in the flue gas are primarily contacted with agglomeration solution fogdrops sprayed in through a dual-fluid atomizing nozzle, agglomeration solution liquid drops are evaporated, in the evaporation process, the PM2.5 particles are connected through macromolecule chains with polar groups, and the grain size of the PM2.5 is increased; the flue gas flows through the turbulent flow area, the agglomeration solution fogdrops are promoted to collide and contact the PM2.5, the PM2.5 are promoted to collide and contact the crude dust under the turbulent flow disturbance generated by a turbulence bar, the PM2.5 are further aggregated and increased in size, meanwhile, the residence time of the agglomeration solution liquid drops in the flue is prolonged, before entering the electrostatic dust collector, the agglomeration solution liquid drops are completely evaporated, and the particles of which the size is increased through agglomeration are finally moved by the electrostatic dust collector. The agglomeration increasing effect of the PM2.5 is obviously enhanced, and the using amount of an agglomeration solution is reduced.