Based on the automatic meteorological station, Doppler radar and NCEP reana lysis data of 1°×1° with 6 h interval, a squall line weather event on July 12 , 2 005 was analyzed. This severe convection occurred under the background of traver se trough developing into vertical trough. When a traverse trough turned into ve rtical trough, the cold air pours from high altitudes, thus an instable stratification with upp erlayer dry and lowerlayer wet was formed, which was carried out by highlo w diffe rential effects of temperature. There was a strong upward airflow before squall line and a falling airflow with large momentum behind it. The surface wind behi nd squall line strengthened by relatively strong momentum transmitted from high to low. When squall line promulgated, old convergence line split or united , so new convergence line came into being consequently. When surface convergence inten sity became stronger, convective weather became more severe. Severe weather was loca ted behind disturbed convergence center. The convergence center was situated beh ind disturbed low pressure band and the high pressure stood before divergence center . There was a low pressure dangled after high pressure, and mesoscale high or low pr essure indicated a foreboding of convective weather.