Abstract: Corn is the crop, which needs much water. The critical stage of moisture metabolism is within 30 days beginning with 8-10 days before blooming. The main obstacle factor in arid and semi-arid area is “dry and lack of water ”. The natural rainfall boom takes place in time of 30 days between Slight Heat and Beginning of Autumn and the moisture supply after corn’s jointing only depends on natural rainfall. Therefore, to determine the optimum sowing time of corn in arid and semi-arid areas, the factor of rainfall boom should be considered as the key to the question, that is, the theory of precipitation-regulating should be applied.