The microphysical characteristics observation test of advection fog was carried out in Tianjin in the winter of 2016/2017.Combined with the droplet spectrum at a height of 66 m above the ground and the atmospheric boundary layer data of 255 m meteorological tower, the development stages of two consecutive advection fog processes after heavy haze were objectively divided by means of change-point detection and trend consistency nonparametric test, which reveals the observation facts of droplet microphysical characteristics and size distribution characteristics at a certain height in the fog, and discusses the evolution law of its generation and disappearance.The results show that with the southwest warm and humid advection, the saturated layer first appears in the air and extends to the ground.During the fog process, the temperature rises within the observation height range in the mature stage, and the fog layer is in a neutral or weakly unstable stratification state.At the height of 66 m, large fog droplets persist, and the microphysical characteristics change quasi synchronously with the ground visibility.The high value of number concentration appears in the early stage of maturity, while the high value of water content and characteristic diameter appears in the late stage of maturity, corresponding to the decrease of fog droplet number concentration and a small jump in ground visibility in the late stage of maturity.In the dissipation stage, the number concentration of each size decreases synchronously due to droplet evaporation.