Abstract: Based on elastic theory and assumed load-transfer function,the differential equations between load-settlement,vertical stress,friction force of pile-side and depth have been derived.The corresponding analytical formulas have also been derived by using approximate solution method-district method of differential equation.The calculation results can image basically the mechanics character rules of the interaction between pile and soil for single pile with cap according to the method of the paper,using directly the relation of load-settlement of pile tests as the known condition.