Mingle,a village of Chagan town in Daan city where all the arable lands are saline soil,and the salinization degree is the most in western Songnen plain,was selected as the experiment site. By researching the evapo-transpiration process of alkali-spot land, the soil water penetrating process, salts movement and water-salts balance laws, the mechanism of saline desertification was then discussed and discovered. The result is that the evapo-transpiration of alkali-spot land primarily is the soil evaporation without plants cover, and salts concentration in surface soil with water evaporation. The soil water infiltration in alkali-spot land is very low, so moved-down salts are much less than those moved up. The salts concentration in surface soil lets the grass landscape degradation, and increases the area of alkali-spot lands, which all exacerbating the saline desertification of this district.