By adopting the advantages of previous theories of laterally loaded pile,in this paper,the authors brought up a 3-parameter theory and deduced its theoretical common solution.At the same time,the theoretical range of n, a very important parameter,was given,too.In comparison with previous theories,the 3-parameter theory's basic differential equation is more common,and its deduction is stricter,and the convergence of its common solution is better.In addition,because for the 3-qarameter theory the idea that every parameter is variable has been adopted,it is able to break through the theoretical restriction of its linear-elastic basic equation.So the 3-parameter theory is essentially different from previous theories and its calculation result is better.