During the night of May 21, 2018 to the day of May 22, severe convective weather process appeares in Yongchuan area of Chongqing. At present, there is a lack of severe convective weather forecasting technology in Yongchuan area. In order to better understand the mechanism of the large scale system and the medium and small scale system in the process of severe convective weather, and improve the ability to forecast it in Yongchuan area. This paper uses dynamic diagnosis method with the routine surface abservation data、sounding data、doppler radar CINRAD/SA data and FY4A satellite infrared C011 image data to analysis the weather system evolution characteristics from May 20 to 22, 2018. The results show that: (1) During the night of May 21 to the day of May 22, the low trough in middle and high latitudes moves eastward, the subtropical anticyclone moves southward slightly, and the system in the middle and low levels confronts each other, forming a stable low vortex with shear line, which constantly stimulate meso-small scale weather system disturbances, with convective clouds developing. (2)And then convective clouds develop into MCC under the convergence dynamics of low vortex and sufficient warm and humid southwest airflow, isolated convection cell developing into squall line, moving eastward and evolving into a bow. The strong winds appeare in the strong echo zone and echo front of the squall line, and heavy rain mainly occurs in the strong echo area and the back of the echo. (3) The atmosphere is in an unstable state of upper dry and lower wet, and the high CAPE index, SI index, K index and LI index show better thermal and dynamic conditions, and strong vertical wind shear appears. At the same time, the vertical wind shear is strong, which is beneficial to coexistence of updraft and downdraft airflow in squall line and the warm moist airflow transported to the developing updraft airflow, then strong wind and short-term heavy rain show. Under the combined action of the synoptic scale systems (the low trough in middle and high latitudes, subtropical anticyclone and low vortex with shear line) and the mesoscale and small scale systems (MCC and squall line), with better thermal and dynamic conditions, strong vertical wind shear and water vapor convergence center appearing, this severe convective weather process forms.