This paper deals with the regularity of annual change of soil moisture in the plain of the Songhua River and the Nen River. The conclusions are as follows: 1. The characteristics of the soil moisture change in the seasonal frozen region are:In the winter, in the area of high ground water table, large quantity of water is transported from the ground water to the frozen layer. In the spring, following the thawing of frozen layer, a water layer lying over the frozen layer is formed and becomes main source of the soil moisture. This process, therefore, deeply influences pedogenesis. 2. The soil water lying over the seasonal frozen layer is, in fact, ground water in the surface soil. The underlying frozen layer is impermeable. The soil water is supplied continously by melting snow, rainfall, irrigation water and water from the thawing frozen layer;it is lost through horizontal run off and evapotranspiration. 3. In the rainy spring; the presence of the seasonal frozen layer is one of the main factor of waterlogging. The salt accumulation in soil is related with the high concentration of salts in the soil water lying over the frozen layer which is near the ground surface. It has no concern with the ground water.