Abstract: Based on the dynamic frame of CM1 model, the necessary conditions for the occurrence and development of severe convective weathers are obtained by theoretical derivation and scale analysis. A typical squall line process under the background of Northeast Cold Vortex (NECV) is selected, and the numerical simulation and comparative test are carried out with a high precision horizontal grid distance of 200 m. The theoretical results are verified and the conditions for the formation of a squall line under the NECV are obtained. Through the scale analysis of model data, it is found that advection, convection and hydrometeor precipitation have the greatest influence on the change of moisture variables in the squall line system, the influence of water vapor phase change is the secondary, and the turbulence effect is relatively small. Thereby, the occurrence of the squall line needs the cooperation of water vapor distribution and the ascending air flow. Simulations over different regions of NECV have verified the results of theoretical analysis. In the southwestern side of the NECV, upper levels are affected by cold air, which, when combined with warm advection in lower levels, leads to the formation of unstable stratification. The unstable stratification and transverse gradient pattern of humidity field can jointly lead to the formation of the squall line. Under the influence of water vapor concentration and temperature, strong radar echoes can easily form in the middle and upper levels with strong updrafts. Under the influence of precipitation, strong radar echoes can also be found in lower levels.