Hygroscopicity is an important feature of ambient aerosols, which is very crucial to the study of light extinction, radiation force, and formation mechanism. The light scattering hygroscopic growth factor ( f (RH)) is an important parameter which is usually measured by the humidified nephelometer system and could better describe the aerosol hygroscopicity under wide particle size range and continuous relative humidity (RH). The f (RH) can be applied to the establishment of a parameterization scheme for light extinction, the calculation of hygroscopicity parameter (κ), and also the estimation of aerosol liquid water content (ALWC). However, the humidified nephelometer system in the previous studies could only observe the f (RH) below 90 % due to the larger error of the sensor under high RH (> 90 %). Furthermore, the f (RH) observations in North China Plain needs to be greatly strengthened both in the temporal resolution and the observation duration. In view of this, an improved high-resolution humidified nephelometer system was established to observe the f(RH) of PM 2.5 for a wide RH range between 30 %–96 % in the urban area of Beijing over three seasons (winter, summer, and autumn) in 2017. It was found that the f (80 %) at 525 nm of PM 2.5 was evidently higher under the polluted conditions and highly correlated with the fractions of all the water-soluble ions. A two-parameter fit equation was selected to fit the observed f (RH) data. For each season, the fitting curve under the very clean condition was lower than that of other conditions. And the f (RH) points of polluted conditions were more concentrated with higher fitting R 2 for summer and autumn data. The hygroscopicity of aerosol under higher RH was probably enhanced when compared with the data in the previous study conducted in NCP. In summer, the fitting f (RH) showed a significant dependence on wavelength for each pollution condition. However, there was an opposite performance in the f(RH) curves of different wavelengths for the very clean condition in winter. It was showed that The simulation showed that the maximum uncertainty of f (RH) was less than 10 %.