Soft interlayer is a widely existing geological body in rock and soil mass. Due to its weak mechanical properties, it usually brings potential safety hazard to engineering construction. Based on the plastic limit equilibrium method, an ultimate-load formulation of soft interlayer is derived by using the modified Prandtl squeezing theory and assuming that the upper and lower layers close to the soft interlayer to be quasi-rigid. In the analysis, we suggests that under the limit state the soft interlayer between quasi-rigid layers displays plastically squeezing failure behaviors. Due to the inaccuracy of Prandtl’s shear stress solution in the x direction, new solutions of stress components and ultimate load of soft interlayer are determined with considering various boundary stress conditions. A simple formulation of the ultimate load on the soft interlayer is derived, which could reflect the comprehensive effects of dead weight, boundary stress conditions, as well as friction condition of boundaries on the ultimate load of soft interlayer. Meanwhile, by comparing results from proposed method to other methods, it is shown that, if the dead weight of interlayer is ignored, the variation of ultimate load with thickness of soft interlayer in this text generally agree with what the Prandtl method predicts; when the dead weight is taken into account, the solutions of the proposed method yields results similar to the method in the construction code. For thick interlayer, however, the proposed method can more properly reflect the influence of width on the ultimate load. In addition, the effect of the boundary friction on the ultimate load varies with width-thickness ratio of soft interlayer, and a critical width is found to exist for the interlayer, which is hardly affected by the friction condition for the interlayer with a certain thickness.