To explore the variation in characteristics of atmospheric pollutants at different stages of haze, the monitor for aerosols and gases in ambient air (MARGA) was used to observe the concentrations of precursor pollutants (NH3, HNO3, and SO2) and eight water-soluble ions in a regional haze in the Yangtze River Delta region from November 18 to December 07, 2018. Combined with environmental data (PM2.5, NO2, CO, and O3) and meteorological data, the causes of regional haze formation, diurnal variation characteristics of air pollutants, and distribution characteristics of air pollutants in different stages of haze were analyzed. The results showed that the Yangtze River Delta region was mainly controlled by a ridge of high pressure during the haze process and the weather situation was stable, which was conducive to the accumulation of air pollutants. On hazy days, the concentrations of PM2.5, NO2, NO3-, SO42-, NH4+, Cl-, and Na+ were (118.91±39.23), (61.62±26.34), (45.64±16.01), (18.80±8.02), (20.82±7.16), (3.02±2.25), and (0.23±0.22) μg·m-3, respectively, and these were 2.73, 1.63, 2.64, 1.94, 2.50, 2.05, and 2.56 times the levels found on clean days, respectively. The concentration of CO was (1.34±0.39) mg·m-3 on hazy days, which was 1.86 times that found on clean days. Diurnal variation characteristics of different air pollutants were different, as were the distribution characteristics of air pollutants at different haze stages. The concentrations of SO2 was the highest in the haze occurrence stage. The concentrations of PM2.5, NO2, NH3, CO, and SNA were highest in the haze development stage, and the concentrations of O3, Cl-, Na+, and K+ were highest in the haze dissipation stage. The relative contributions of SNA to PM2.5 in different stages of haze could reach 94%-96%, and their growth rate was largest in the development stage. The order of growth rate was NO3- > NH4+ > SO42-. SNA mainly existed in the form of NH4NO3 on clean days and in the occurrence and development stages, and (NH4)2SO4 in the dissipation stage. This haze process was mainly caused by the growth of NO3-, which was mainly generated by gas-phase homogeneous phase reaction, and NO3-contributes 51.06%, 51.85%, and 48.22%, respectively, to PM2.5 in the occurrence, development, and dissipation stages of haze.