Abstract: The authors have experimentized the wind velocity of threshold sand under the rainfall condition of 0.1, 0.2, 0.3, 0.4, 0.5, 1.0 and 2.0 mm in the wind tunnel -with rough and fine sand samples. The results have shown that micro-rainfall could evidently increase the wind velocity of threshold sand, the more rainfall , the more wind velocity of threshold sand, the rainfall of 2.0 mm can resist hurricane in a short time. The authors have discovered that the rough sand has more resistance to wind with the same rainfall in both rough and fine sand samples. The various destroy of two sand samples has discussed in this paper. when the wind velocity has increased large enough, the humid sand surface of fine sand has been suddenly partial turned over, under which the dry sand layer was exposed, then being rapidly hollowed out and destroyed. But the humid sand surface of rough sand was not able to turn over at once under the effect of wind force, the sand grain began to creep after the surfce was blown and dried. when the humid sand lyer was thinned enough, it would be turned over. The authors also have probed into the destroy physical mechanism of both rough and fine humid sand layers, and proposed that there were different shearing strength between humid sand and dry sand. When the shesring stress was come up to the shearing strength of humid sand and dry sand boundaries, the surface of humid sand would be turned over. Otherwise, it hasn't been turned over, but being slowly evaporated, dried, crept, thinned, unless the shearing strength was reduced to equalization of wind shearing stress.