The softening effect of the smoothed finite element method in stiffness is first verified in theory as the element is consecutively divided into subcells. Next, three kinds of quadrilateral elements are investigated, including those with mapped iso-parametric shape functions, with the global polygonal shape functions and with the piecewise triangular shape functions, to validate the softening effect and the required conditions for shape functions. The calculated stiffness values show that the softening effect is only suitable for the element when the shape functions are not only well defined over the whole element but also remained in this sub-division process.