Abstract: The soil water characteristics are the basis for developing crop irrigation system. In order to formulate reasonable tobacco irrigation system and reduce the seasonal drought impact, this thesis studies the typical physical properties of purple soil and soil water characteristics in Nanxiong City. Through field and laboratory tests, the authors analyzed the soil moisture characteristics under different rotation type and different depth of purple soil. The results showed that the saturated water content of the purple soil of Nanxiong dry sloping field was 30.7%, the field capacity was 25.5%, wilting co-efficient was 10.0%; the soil’s water retention was poor and effective moisture percentage was low, because 30% of the soil water would loss by the effect of gravity and more than 50% of the soil water could not be used by crops. The results showed that different crop rotation’s effect on moisture characteristic curve was slight, but the influence of soil depth was larger. It was advised that the local departments should pay attention to frequent scarification or add rice straw to increase water retention of the soil in production.