Using conventional observational data ,satellite images, Dopper radar echo and NECP data, the causality of the strong convective severe sand dust storm in the south of Hebei on May 10,2005 is discussed. The results show that the v ertical structure of cold advection in the upper air and the warm advection in t he lower level, and increasing temperature at afternoon provided the advantageou s thermal condition. Strong surface winds were mainly caused by a squall line ah ead of the cold front which showed as a arc echo, the surface mesoscale converge nce line is very important to the squall line. Meanwhile, the development of upp er level jet and strong downward transport of momentum by the sinking branch o f the upper level jet's secondary circulation are also important for the formatio n of strong surface winds. The dust devil occurred at the coupled area of high/l ow level jets and the superposition of the positive helicity value at 700hPa and highenergy tongue and the higher gradient of the helicity's positive value at 300hPa, the magnitude of the tropospheric helicity is useful for the prediction of dust devil events.