In order to know efficiently the impact behavior of the actual size sand cushion under rock fall, to achieve a lot of impact experiments under various conditions or to analyze those phenomena by use of reliable accurate methods is generally required. However, there is difficulty in many cases to achieve experiments because of financial reason. Therefore, the analysis method to reproduce full-scale impact behavior is indispensable. From this point of view, we have developed Distinct Element Method. The method to model the body to collide like a rock fall by an assembly of many small elements and the method to present a sand cushion as an assembly of many elements which have different size with randomness were shown. The validity of this method was shown by the comparison with experimental results.