During 4-5 April 2018, an infrequent snowstorm stuck Beijing. This paper analyzes the extremes and formation mechanism of the blizzard process by using multi-source observation data such as raindrop disdrometer, Doppler weather radar, microwave radiometer, ground encryption automatic station, EC reanalysis data and climate reorganization data. The results showed that (1) the 5th of April 2018 was the first pure snow day in Beijing in April. The snowfall and snow depth both exceeded the historical record and the 1000-850 hPa temperature normalized SD value was less than -3. So, it was an extreme weather process. (2) The invasion of low-level cold air forced the warm and humid air to climb along the cold pad. Conditional symmetric instability from 800 hPa to 500 hPa in Beijing Area was caused by strongly warming and humidification from 700 hPa strong southwest low-level jet and triggered by the strong ri-sing motion of warm air above the front, resulting in elevated convection and radar echo like thunderstorm cells in summer, which enhanced the snowfall. (3) The snowfall process was affected by two cold air superimposition successively. The continuous violent cooling induced by a strong cold air before the snowfall caused the low-level temperature to be extremely low and reached the temperature threshold of snow in Beijing, which was the decisive factor for the extreme snowfall. (4) Microwave radiometer monitoring showed that the duration of snowfall had a good correspondence with the temperature inversion, and the precipitation phase mainly depended on the temperature change below 1 km.