A small remote-controlled Unmanned Aerial Vehicle(UAV) carrying meteorological and environmental monitoring equipment was used to observe the PM_2.5 concentration, temperature, relative humidity and other elements at different altitude levels in Taiyuan from July 29 to 31,2020. According to the flight experiment results, the three-dimensional distribution of PM_2.5 and its variation characteristics over time were obtained. The causes of pollution accumulation and dissipation, the influence of external transport were analyzed through the vertical distribution of PM_2.5 concentration and meteorological elements, the weather situation and backward trajectory analysis. Results show that compared with the horizontal distribution difference, the vertical distribution difference of PM_2.5 concentration is more significant. Most of the time, PM_2.5 concentration meets the rule of "higher-bottom and lower-top", and the hygroscopic growth of particulate matter may lead to the situation of "lower-bottom and higher-top". The relative humidity generally increases with the increase of height, but the temperature varies in the opposite way; the inversion layer inhibits the vertical mixing of atmosphere obviously. On 29, there was a process of pollutant accumulation. On 30 and 31, there was a process of pollutant dissipation in the daytime and an accumulation process in the night. The warm and wet airflow brought by the subtropical high and the weak wind near the ground were the reasons for the gradual accumulation of particulate matter on 29. On 30, the relative humidity dropped rapidly, and the wind speed was relatively high, which was conducive to the decrease of the concentration of particulate matter. On 31, the ground relative humidity was the lowest, and the strong development of upper and lower convection led to the rapid decrease of the PM_2.5 concentration. During the three days, the air flow mainly came from the southeast direction. The local accumulation of PM_2.5 in Taiyuan was dominant, while the non-local transmission was mainly from neighboring cities such as Jinzhong and Changzhi in the province.