Proceeding from the soil microscope statistics and by using the statistics capillary bunch model developed by the pore grading function with isotropic, homogeneous and unstructural "soil slice" and the theory of mechanics depending on the soil moisture distribution, this paper proves the mechanism and law of moisture functions (such as hydraulic conductivity, matrix potential and diffusivity) in Richards Equation and the infiltration rate as well as their relation, which change with saturated degree and soil matrix. This paper also proposes the corresponding calculation equation and the determining approach on the parameter analysis for practical operation,promoting the existing pure empirical correlation about the study of moisture function so far to the level of theoryanalysis and made the establishment of theory formula into reality. The new infiltration formula, which contains soil moisture factor before infiltration, can replace Philip's infiltration formula whose truncation error is unconverged. With the experiment result on Sarpy loamy soil and Geary mealy loamy soil listed in reference [1],the conclusion is verified.