Abstract: In order to systematically analyze the emission characteristics of PM_2.5 at the outlet of wet desulfurization system(WFGD), based on the on-site test of the particulate matter(PM) and PM_2.5 at the exit of WFGD in 26 ultra-low emission units, the effects of inlet PM concentration, flue gas flow rate, liquid gas ratio, pH value and droplet concentration on the PM_2.5 emission at the outlet of WFGD are investigated. The measurement results indicate that the proportion of PM_2.5 to PM surges with the decrease of mass concentration for PM. PM_2.5 emission coefficient increases with the increase of flue gas flow rate, liquid gas ratio, pH value or droplet concentration for absorption tower. The increase rate of PM_2.5 emission coefficient is higher than the increase rate of flue gas flow rate, liquid gas ratio and pH value, but it is basically consistent with the increase rate of droplet concentration. Which shows that droplet concentration has a direct impact on PM_2.5 emission coefficient, ant the key to reduce PM_2.5 emission factor is to control droplet concentration in WFGD.