This　study　presents　a　comprehensive　design　and　calculation　methodology　for　prefabricated　expansion　foundations　of　onshore　wind　turbines,　addressing　the　critical　need　for　modular　construction　while　ensuring　structural　integrity.　The　proposed　approach　encompasses　both　overall　structural　performance　verification　and　connection　joint　design,　considering　the　stiffness　reduction　effects　of　modularization.　A　novel　punching　shear　capacity　checking　method　is　developed　specifically　for　assembled　expansion　foundations,　incorporating　joint　weakening　factors.　The　research　establishes　equilibrium　equations　and　verification　formulas　for　splice　joints　under　different　loading　conditions,　demonstrating　that　the　shear　amplification　factor　at　joints　varies　with　local　compressive　stress.　The　analytical　results　demonstrate　several　key　findings:　(1)　the　derived　maximum　joint　shear　formula　shows　excellent　agreement　with　finite　element　results　across　various　load　conditions　(R2　=　0.99906);　(2)　the　optimized　shear　key　configuration　increases　the　joint＇s　load-bearing　capacity　by　35%　compared　to　conventional　designs;　and　(3)　the　developed　Python-based　calculation　system　reduces　the　design　time　by　60%　while　maintaining　accuracy　within　5%　of　detailed　FEM　analysis.　These　quantitative　outcomes　validate　the　effectiveness　of　the　proposed　methodology　and　provide　practical　guidelines　for　implementing　modular　construction　in　wind　energy　infrastructure.