Applying combined surface,the graphical method is used to calculate the passive earth pressure usually. Not only procedure of the graphical method is complicated but also its precision is poor. Based on the principle of the circular combined surface and methods of mathematics and mechanics method ,a formula of passive earth pressure on cohesive soil,which can be applied to the trial calculation by programming procedures,is proposed. The case analysis shows that the calculated results are the same as those calculated by the Coulomb theory under the conditions of the smooth vertical wall back; In the small,there is slight difference between the two; But with the increase of,the calculated results of Coulomb theory are significantly greater than those calculated by the method. Therefore, in a greater external friction anglebetween the wall and the backfill, it should use the method which optimizes the engineering design and reduces project cost.