Through references and comparison of main current computations for stability against overturning of cellular steel sheet pile, a new calculation method“, modified separation method”, is raised:assuming that the gravity of the packing inside the cellular sheet does not directly participate in anti-overturning, instead, the mechanism and degree of anti-overturning are represented by the lateral pressure and friction of the soil against the wall of the cellular sheet, accordingly amendment is made to the traditional“TVA separation method”, and meliorated calculation method is provided. Moreover, though a case study, the result is compared with those of the current five methods in order to verify the rationality and practicality of the present one.