One long lasting squall line storm that struck North China severely in the afternoon of 23 June 2008 is analyzed based on various observation and analysis data collected by radar, satellite, and auto weather stations, etc. This servere convection developed in the background of cold vortex in summer, and an isolated MCS evolved near the sub cold front. Strong convective cells and several super cell storms were active in premature period of the squll line, and the organization of convection is attributed to the surface wind convergence line or front. After the main cold front left at the night of 22 June, the easterly and southerly wind rearside of the high pressure transported warm moist air into the increasing intensity of the frontal zone. On the other hand, from the afternoon of 23 June, a short wave trough near the Hetao Area with high relative humidity developed in the mid and lower levels of troposphere, corresponding to the sub cold front moving towards North China and conecting to the initiation of the convection, and the updraft motion due to the front was diagnosed by the secondary circulation equations. In the rearside of the cyclone, the dry and cold air with obvious subsiding made strong potential instability which is favorable for the convection initiation and formation of strong surface wind. By analyzing the vertical wind profile and the movement of squall line, it is indicated that the boundary moves approximately at the same speed as the lower level vertical shear, so MCS maintains the strength and the lasting time.