A dynamic methodology was developed to characterize the surface activity of water soluble humic-like substances (HULIS) in PM_2.5. As a case study, HULIS in PM_2.5 samples collected from a rural site during wintertime in North China Plain region were investigated. The HULIS carbon content varied from 2.0~4.6μg C/m³, accounting for 31%~40% of water-soluble organic carbon and 20%~26% of organic carbon, respectively. While the carbon content of HULIS aqueous solution ranged from 88~200mg C/L, correspondingly, the surface tension reduced by 18%~22% compared with that of pure water. The surface tension decreased significantly when the concentration was lower than 70mg C/L, while it decreased slowly between 88~320mg C/L. The dynamic measurements showed that surface tension gradually decreased with time consuming. Surface tension decreased rapidly within 200 seconds after droplet formation and then tended to be stable, indicating that the distribution of surface-active organic molecules onto the droplet surface was not instantaneous. Such process may affect droplet activation. There were substantial surface-active substances in atmospheric particulate matters in the polluted atmosphere. These surfactants may have significant impacts on the activation of aerosol particles into cloud or fog droplets. In addition, the HULIS may lead to liquid-liquid phase separation and form the core-shell structure of particles when ambient relative humidly fluctuates. Such structure could influence the uptake of reactive molecules and the gas-particle partitioning, therefore, affecting heterogeneous chemistry.