A squall happened on 23 July 2004 in North China is analyzed on the basis of conventional meteorological observation data, as well as cloud, automatic weather station and Doppler radar data. It is found that the squall took place in the instability area near the northwest of the subtropical high, and was caused by a mesoscale convective system (MCS). A surface cold front, the southward moving of the subtropical high, and the upperlevel forward trough made the MCS move southeastward and enforced into a squall. The low and middlelevel moisture flux convergence in North China provided plenty of water vapor for the occurrence of strong convection. The lowlevel jet at 850 hPa and 700 hPa brought warm and moist southwest air to North China and intensified the unstable stratification. The strong wind shear near the lowlevel jet provided dynamic conditions for the occurrence of the squall.