During the treatment of subcostal lesions with high intensity focused ultrasound (HIFU), the obstruction by the ribs significantly affects the therapeutic effect, which can be assessed through numerical calculations. In existing studies, ribs are typically regarded as perfect acoustic absorbers, and even though this assumption could reveal the influence of the ribs on the acoustic field to some extent, it may still underestimate the energy behind the rib cage. In order to address the shortcomings of current work, an innovative numerical calculation method that avoids treating ribs as perfect sound absorbers is proposed in this work. Subsequently, experiments are conducted using ABS plastic rib cage mimic to compare the effectiveness between the two methods, demonstrating that the method proposed in this paper, which avoids the assumption of considering ribs as perfect acoustic absorbers, could better reveal the influence caused by ribs, and further studies are carried out on the influence of ribs in a multi-layered medium model. In response to the numerical oscillation issues encountered in existing work when dealing with media with high acoustic attenuation coefficients, the operator splitting method to enhance the stability of numerical calculations is adopted in this work. Furthermore, to tackle the challenges posed by asymmetric acoustic fields in numerical computations, in this paper matrix vectorization technique is introduced and stable solutions for the acoustic field under the backward implicit difference scheme are obtained. Additionally, when considering nonlinear effects, an asymptotic maximum number of harmonics is employed to reduce the computational load. These improvements in both the numerical calculation model and the corresponding algorithm not only enhance the precision of numerical computations, but also reveal the underestimation of energy behind the ribs due to the assumption of perfect acoustic absorbers, which is significant for optimizing HIFU treatment strategies.