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The largescale strong convective storm and the local hail storms over Ningxia, Inner Mongolia, Shaanxi, Shanxi and Henan on 15 and 16 June 2004 are analyzed by using the potential vorticity method. The spatial structure of dry potential vorticity (PV) exhibits that the positive largevalue potential vorticity center inclined to west from upstream to storm zone; the positive potential vorticity center in the upper troposphere and the dense PV isoline zone with enhanced PV gradients in the lower/middle troposphere were overlapped in the storm area. The dry PV field in the lower troposphere shows that the coupling of the dense PV isoline zone and the wind shear was formed in the storm area. The wet potential vorticity field in the lower troposphere shows that the coupling of convective unstable tongue and the wet baroclinicty center and the dense isotimic area of MPV2 was formed in the storm area. Before the storm, the dense meridional PV isolines was formed over Hetao in 500 hPa, and the positive PV disturbance and the positive PV advection toward Hetao appeared at the same time in the strong north flow over the Mongolia area in 500 hPa, which is highly indicative for the occurrence of largescale strong convective storms on the Loess Plateau.