Based on the intensive regional automatic meteorological station data, wind profiler data, routine observations, and NCEP reanalysis data, the circulation background, dynamical, and thermal conditions, especially the atmospheric stratification, are analyzed for the continuing severe convective weather during the onset period of 2014 rain season in Guangdong province. The results show that the severe convective weather appeared in the severe convergent area and updrafts in front of the maximum wind speed of a lowlevel jet, and the vertical wind shear increased rapidly before the severe convective weather happening, which has indicative significance for prediction. The mesoscale depression and ground convergence line are indicative of the convection location. Thermodynamic analyses show that the lasting warm advection by lowlevel jet along the temperature gradient from the warm area of Beibu Gulf is the fundamental cause of the maintenance of the instable stratification in Guangdong Province. Therefore, the variation of lowlevel jet and temperature field should be paid more attention for the severe convective weather during the onset period of the rain season in Guangdong Province in forecasting.