Separation of fine particles from coal-fired flue gas by vapor heterogeneous condensation was investigated experimentally in a wet flue gas desulfurization (WFGD) system using a cascade of double towers. The supersaturated vapor environment can be achieved in two ways. The influences of flue gas relative humidity and the temperature difference between the flue gas and the desulfurization liquid at the inlet of the main tower on the collection efficiency were investigated in this paper. The results show that high collection efficiencies can be obtained by this separation process and the collection efficiency is higher than that of single tower WFGD system. Flue gas relative humidity at the inlet of the main tower can influence the separation of fine particles significantly. The collection efficiency is improved significantly by injecting steam among the rotating-stream-tray shutters and optimizing the temperature difference between the inlet flue gas of the main tower and the desulfurization liquid. The influences of temperature difference on the separation of fine particles include the following: the collection efficiency decreases with increases in the temperature of the desulfurization liquid (T-L) in lower temperatures than that of the flue gas at the inlet of the main tower (T-G), and the influence trend is contrary while T-L is higher than T-G. Particle collection efficiency also increases with increasing liquid/gas ratio in the main tower.