Abstract: The physical mechanism of a black storm, which burst out in Northwest China on May 5, 1993, is studied using the methods of synoptic meteorology and diagnostic analysis. The results show that the black storm is a mesoscale phenomenon which forms and develops in particular season (spring) on the bare loose underlying surface (desert, gobi) when a strong cold wave bursts out. The results also show that there exist strong wind shears in the vertical direction of middle and lower convective layer; the wind shears are favourable to the development of turbulence; the contribution of the non-geostrophic forced function to the secondary circulation is greater than that of the geostrophic forced function. The positive thermodynamical feedback, resulting from the significant differential radiative heating before and after the appearance of the black storm, leads to the local frontogenesis of surface layer, and strengthens the positive thermodynamical circulation.