This study investigates the morphological variations in 500kV XLPE submarine cables and factory joints, analyzing their impact on dielectric properties and AC breakdown strength. Samples from the cable bulk, cable-joint transition, and factory joint were examined using extraction tests, FTIR, DSC, and XRD. Results show that factory joints exhibit higher crosslinking byproducts, sparser crystal structures, and lower crystallinity compared to the cable bulk, leading to reduced AC breakdown performance. These findings provide critical insights for optimizing XLPE insulation morphology, particularly in submarine cable factory joints.