Objective To evaluate the effects of different restoration methods and post-core materials on the mechanical properties of residual roots of mandibular second premolars. Methods Four restoration models (NPR, CLP, OEP, ISRF) were established by three-dimensional finite element method, combined with three types of post-core materials including glass fiber post, titanium alloy post and nickel-chromium alloy post, respectively. A static load of 400 N was applied obliquely at 30° along the long axis of the tooth. The stress and displacement of different restoration methods, as well as the biomechanical behaviors of different post-core materials were analyzed. Two-way analysis of variance was used to evaluate the main effects and interaction effects of restoration methods and post-core materials on various stress and displacement indexes. Results The equivalent stress of abutment teeth, maximum principal stress of adhesive interface, equivalent stress of intracoronal adhesive and maximum displacement in the ISRF group were lower than those in the NPR, CLP and OEP groups (P<0.05). The maximum principal stress inside the post increased with the increase of elastic modulus of post-core materials. Among them, the glass fiber post showed the lowest stress level in all observation models, with the maximum principal stress inside the post being (25.36±1.42)MPa, which was lower than that of titanium alloy post [(38.74±2.15)MPa] and nickel-chromium alloy post [(45.83±2.54)MPa] (P<0.05). Both restoration methods and post-core materials had significant main effects and interaction effects on various biomechanical parameters (P<0.01). Conclusion The application of ISRF combined with glass fiber post in the restoration of mandibular second premolar residual root can effectively reduce the peak stress of abutment teeth and adhesive interface, and improve the overall stress distribution. In addition, the ISRF structure also plays a certain buffering role in material sensitivity, showing good elasticity and adaptability in terms of mechanical stability and clinical application.