PM 2.5 is generally considered as a main pollutant causing the formation of haze. Based on meteorological parameters, aerosol distribution, and PM monitoring data in Taiyuan during November and December 2016, the characteristics of the key size spectrum of PM 2.5 affecting haze were discussed. During the observation period, haze was frequent and serious. Heavy haze time accounts for 25.35% of the total haze time. Haze events occurred frequently when the relative humidity was greater than 80% and wind speed was less than 1.5 m·s -1 , especially for severe haze. Mild and moderate level haze occurred frequently when the relative humidity was less than 80% and greater than 40% and when wind speed was less than 1.5 m·s -1 . Slight haze mainly occurred when the relative humidity was 20%-40% and the wind speed was 1.25-2.55 m·s -1 . The average mass concentration of PM 2.5 was 209.45 μg·m -3 , which was three times the level during non-haze events. With an increase in the haze level, the mass concentration of PM 2.5 and the ratio of PM 2.5 /PM 10 increased. PM 1 was the key particle size affecting haze in the low humidity environment. PM 0.5 was the key particle size that affects slight haze, mild haze, and moderate haze in the high humidity environment, while PM 1 was the key particle size that affects heavy haze. The contribution of surface concentration to visibility decreased with high humidity, but the particle size increased by moisture absorption leading to an increase in the extinction efficiency factor, which compensated for the lack of surface concentration. The increase in the particle size parameter was an important factor for PM 2.5 affecting the haze pollution with high humidity.