Using the observational and NCEP/NCAR reanalysis data, the characteristics of an unusual severe summer sand-dust storm in Hexi corridor is analyzed. The results show that upper trough and shear line and thermal depression are main synoptic systems introducing summer sand-dust storm. The value of air pressure, wind speed and relative humidity increased, and the value of air temperature descended when the summer sand-dust storm occured, but the changing extents were less than that of meteorological elements in spring sand-dust storm. The summer sand-dust storm is as characteristics of middle-small scale cloud cluster on cloud image, the cloud cluster of TBB≤-35℃ can show the change of sandstorm in summer. The diagnostic analysis shows that there is convergence in low level and divergence in upper level, sandstorm happened after the appearance of max vertical velocity, and at the same time, the helicity in eve of sand-dust storm eruption has good indication in the forecasting of sand-dust storm. When the helicity is large, the sand-dust storm is severe.