The stability calculation of overlength piles is a very important problem. In this paper the soil resistance mode with two parts in the lateral side of pile is proposed. The m-method mode is for shallow soil and the constant method mode is for deep soil. Rely-Ritz method is applied to analyze the stability of overlength piles. The critical load formula for overlength piles is obtained. The variation law of length coefficient of overlength piles is researched so as to provide a theoretical reference for overlength piles in engineering practice.