Abstract: A basic eguation for predicting 1100 mb topography is obtained on the following two assumptions: (1)the level of P=0 is a horizontal plane,(2)its height H does not change with timo. This equation is very similar to that for barotropic case, but with certain essential differences. In this article, author also points out that in the barotropic case, taking a smaller grid we will get a better rosult than that of alarge grid. The oxamples of Forecasting shove that this method is rather successful.