Reflective cracking is one of the primary early-stage distresses in asphalt overlays on existing cement concrete pavements (white-to-black resurfacing structures). To delve into its mechanical evolution mechanism, this study established a three-dimensional thermal-mechanical coupled numerical model using the finite element software ABAQUS and quantitatively analyzed the variations in the stress intensity factor (SIF) during crack propagation using the J-integral method. The study systematically investigated the influence of material modulus, structural layer thickness, and external loads (traffic and temperature) on the SIF. Sensitivity analysis results reveal that environmental temperature drops and vehicle overloading are the core drivers of reflective crack propagation, with significantly higher sensitivity than structural material properties. Increasing the thickness of the asphalt overlay generates a notable “bridging anti-cracking” effect, representing the most effective structural means to inhibit crack propagation. The research findings provide a theoretical basis for anti-cracking design and lifespan prediction in the rehabilitation of existing pavements.