Laser cavity ring-down spectrometers (CRDS) developed are used to investigate the aerosol light extinction and the influence caused by relative humidity during a long term experiment around Kunshan, Jiangsu. Observation results show that mean value of extinction coefficient is quite high, and a clear diurnal pattern is observed in extinction coefficient with maximum occurring at 7:00, dropping down to minimum at 17:00, then rising again, which is caused by the great influence of the surrounding heavily polluted areas and meteorological conditions. Results for the hydrophilic properties of atmospheric aerosol in the monitoring site show that atmospheric aerosol is hydrophilic type, and extinction coefficient is susceptible to the ambient relative humidity. The results have certain guiding significance for the further study of atmospheric aerosol properties of this area with aerosol mass spectrometry and other monitoring instruments.