The invention provides a method and a device for promoting simultaneous removal of mercury and fine particulate matters during wet flue gas desulfurization by use of the water-vapor phase change principle and aims to overcome the shortcomings that flue gas in low in temperature and high in humidity and PM2.5 and mercury cannot be effectively removed after wet desulphurization. Coal-fired flue gas subjected to wet desulphurization enters a water-vapor phase change chamber, and normal-pressure saturated vapor is injected to enable flue gas to be supersaturated; on one hand, the supersaturated water-vapor drags gaseous mercury to be collected on the surfaces of particulate matters through both the thermophoresis and diffusiophoresis actions, and on the other hand, fine particulate matters on which particulate mercury is collected are taken as condensation nucleuses, nucleation and condensation occur on the surfaces of the condensation nucleuses, so that the fine particulate matters are increased in particle diameter and mass, and dust-containing drop mercury which is increased in size by condensation is finally removed by an efficient demister. A conventional coal-fired wet desulphurization device is improved by additionally arranging the water-vapor phase change chamber behind a desulfurization tower, so that once vapor is added, the wet desulphurization device can play the role of simultaneous removal of mercury and fine particulate matters. The method and the device can be widely used for wet flue gas desulfurization in coal-fired power plants and have huge economic benefits and social benefits.