The limitation of the current calculating formula is pointed out based on the analysis of the loading mechanism of composite ground. The pile is divided into two situations based on the pile soil limiting relative displacement, namely plastic stage and elastic stage. The two parts are calculated by different methods. The friction of plastic part is calculated by effective stress method which considers the confining pressure, and the friction of elastic part is calculated by load transfer method. A formula for calculating the skin friction is obtained, and accordingly the depth of neutral point and the pile soil stress ratio should be found. Lastly, an engineering example is analyzed by this method, and the calculated results agree well with the field measured data.