A new technique named continuous dynamic grid adaption (CDGA) technique is introduced in this paper.In order to improve the accuracy of numerical solutions by making arbitrarily local grid close together and to avoid great increase of computational expense. Considering variable principle, the resulting mesh from CDGA technique has advantages of smoothness, orthogonality and the adjustmentability in cell volumes. Using the MM4 with CDGA technique, the simulation results of the heavy torrential rain event on June 14,1992, show that the modified model runs steadily, and the 24 hours rainfall forecast is evidently improved.