Abstract: On 5 May 1993,a dark storm (strong sandstorm) recorded rarely in the history took place in northwest of China and took a large toll in human life and property damage .This event brought on general attention in the world .The meso-scale squall is an important influence system of "5.5" dark storm .By using of improved PSU/NCAR mesoscale model with 15-km grid interval,the operational rawinsonde observations as initial data,we simulated successfully the main characteristics of the squall,including surface wind shear-line,storm center related with the squall,strong and narrow convective zone,strong horizontal temperature and pressure gradients as well as the time of their formation and rapid reinforcement and moving direction .The vertical structure of the simulated squall revealed that the largest temperature gradient and vertical velocity are confined at 700hPa and vertical circulation sloped north-westward with height .The strong cold advection downdraft in the rear of squall brought by the short wave disturbance at mid and low levels is an important mechanism for the development of the squall line.