This paper deals with the problem of secondary salinization of soils caused by irrigation with bitter water in the arid and semiarid regions of southern Ningxia. Sierozem is widely distributed in this region. For development of agricultural production, it is necessary to irrigate with bitter water due to lack of the fresh water in this region. After irrigation with bitter water containing a large amount of soluble salts, the soil secondary salinization occured immediately by which the crop yield was affected. Some conclusions obtained from the study on the dynamic regime of water and salt in soils irrigated by bitter water are summarized as follows: 1. It was observed that the lower limit of soil moisture for the water movement in light loamy soil was about 10% (by dry weight).If the soil water content was above 10%, it could dissolve the salts and carry the salts to move up and down in the soil profile. If the soil water content was below 10%, it could not dissolve the salts. 2. From the view point of long term irrigation with bitter water, the light loamy soil with good filtration and high permeability is suitable to be used for irrigation with this water, although it has low retention capacity of water and nutrients. Under the condition of the ground water level more than 20-30 m in this region, the irrigation can leach the harmful salts downward from the upper soil, which will be beneficial to improve the saline soils. On clay soil, irrigation with bitter water is more successful in the initial stage due to economy of irrigation water. however, owing to the poor permeability of the clay soil,,long term irrigation with bitter water inevitably firings about accumulation of salts in soil and hence servere secondary salinization of the soil. Therefore, the area of clay soil is not suitable to be used for irrigation with bitter water. 3. It was proved that the secondary salinization of the soils irrigated with bitter water in the initial stage was mainly caused by the salts brought up by the capillary water from the subsoil containing a large amount of salts, while the salt carried by irrigation water was insignificant on the secondary salinization of the soil. 4. The movement of water-salt in the soil mainly depends upon the seasonal variation of climatic factors such as temperature, rainfall, evaporation, etc. According to the annual variation of salinization and desalinization in soil, it is divided into four periods in a year:(1) period of violent salinization, (2) period of desalinization caused by irrigation, (3) period of the natural desalinization and (4) period of slow accumulation of salt.