The mesoscale meteorological model (MM5V3.7) is used to simulate a strong sand and dust storm (SDS) process in North China from 18 to 22 March 2002. With the high resolution data from MM5V3.7, the PV theory (potential vorticity) is applied to reveal the mechanism of downward momentum transport. The results show that: (1) the Mongolia cyclone was the main weather system during this strong SDS process, and gales and dryness were the preconditions of the wind erosion; (2) the downward transportation of potential vorticity along the iso entropy surface was the most important process during the SDS, which clearly reveals the mechanism of downward momentum transport.