Abstract: Characteristics of moist potential vorticity fields when elevated duct appears are simulated with the numerical Mesoscale Model 5 (MM5) based on National Centers for Environmental Pridiction(NCEP) final operational global analysis(FNL) data and sounding data. Simulated result shows that the elevated duct region occrus in the center that negative moist potential vorticity fields changes rapidly. Moist potential vorticity is a combined parameter that denotes dynamics and thermodynamics process, and can be used to mechanism and forecast research. With its impervious property, moist potential vorticity can only move along isentropic surface, and its negative center and gradient acutely augmenting regions can only move along potential equivalent temperature surface too.Since move velocity and direction of the moist potential vorticity are knowable, the move velocity and direction of elevated duct are knowable and forecastable, which presents a new way to elevated duct forecast.