Based on the observation data and ECWMF 6hour reanalysis data and the Southwest Regional Numerical Prediction Model, a very severe snowstorm occurred in the middle east of the Western Sichuan Plateau in 21 February 2016 is analyzed and diagnosed. The results show that the snowstorm was mainly related to the southwest jet and the frontogenesis formed in the combination with the cold air and southwest warm and humid airflow. The cold air to the south of the horizontal trough of Baikal was incorporated into the low trough in the central Western Sichuan Plateau. The Bay of Bengal and South China Sea are the sources of the vapor of the snowstorm as the subtropical high moves northward. The heavy snowfall occurred when Slantwise Vorticity Development (SVD) strongly developed and the snowstorm area was the same as the strongest zone for SVD development. The Southwest Regional Numerical Forecast Model made the accurate forecast 6 hours in advance of the snowstorm’s potential field and physical field. The precipitation forecast was one order of magnitude weaker than the actual precipitation, and the falling zone forecast of the snowstorm was more accurate than its strength forecast.