This study systematically investigated the microstructural evolution, mechanical property mismatch, and their effects on fatigue performance in laser-remelted nickel-based superalloys through multiscale experimental characterization and finite element simulations. By combining nanoindentation tests, fatigue experiments, and an improved indentation inverse analysis algorithm, the spatial distribution of mechanical parameters in the remelting zone (RZ), remelting-affected zone (RAZ), and base material (BM) was quantified. The results demonstrate that the RZ, characterized by coarse columnar grains and Laves phase formation due to non-equilibrium solidification, exhibits significantly reduced strength compared to the BM. However, dendritic-cellular substructures within the RZ mitigate macroscopic performance anisotropy through grain boundary pinning effects. Although the RAZ shows elevated geometrically necessary dislocation (GND) density and twin boundary proliferation, residual tensile stresses lead to underestimated nominal hardness measurements. Fatigue analysis reveals a stress shielding effect in the heterogeneous material system: cyclic softening and mean stress relaxation in the RZ under high-stress conditions substantially reduce crack driving forces, shifting fatigue failure initiation to the BM. This work establishes a cross-scale theoretical framework for optimizing the performance of laser-repaired components.