Soil water retention curve refers to the relationship between volumetric water content and pressure head,which plays a very important role in solving the problems of water flow and solute transport in soil. Due to its complexity,it is difficult to derive an accurate equation theory erically However,quite a number of experience formulas have ever been proposed on the basis of a number of experiments,such as Brookscorey(1964) model,Gardner(1970) model,van Grnuchten(1980) model,and Gardner-Russo(1988) model etc.,in which many parameters need to be determined.In this paper,by means of the least square method,the nonlinear equation groups that are used to obtain the parameters in models above mentioned, are derived respectively,and their Picard iterative forms are got.At last,the suitability of the four models for different tvpes of soil is indicated by the numerical examples.