Abstract: Based on many kinds of observations, including conventional data, cloud image, Doppler radar data and wind profiler data, a squall happened in the Yangtze River Delta on 24 August 2002 is analyzed. It is found that this squall was caused by a mesoscale convective system (MCS). Cold front, water vapor transportation beside the subtropical high and a 500 hPa trough made the MCS arise and enhance. The fading and moving southward of the subtropical high and the local advantaged dynamic and air-layer conditions caused the MCS expand to be a squall, and the squall moved rapidly southward and eastward and brought wind and thunderstorm. The squall’s echo and wind structure are well figured out by Doppler radar, auto-station and wind profiler data .