Extruded crosslinked polyethylene (XLPE) submarine cables with factory joints play a significant role in cross-sea transmission interconnection projects. Morphological variations in the insulation of submarine cables and factory joints are caused by differences in process parameters. The dielectric properties of XLPE are affected by its morphology. In this study, samples of XLPE insulating materials were taken from the cable bulk, cable–joint transition, and factory joint, respectively, of a 500 kV high-voltage AC submarine cable. The crosslinked morphology was obtained by extraction and infrared spectrometer. The crystal morphology was obtained by the way of etchant, differential scanning calorimetry, and X-ray diffractometry. The Weibull distribution was used to gather statistics of AC breakdown strength. The results show that in comparison with the cable bulk, the contents of crosslinked byproducts are higher in the cable–joint transition and factory joint. At the same time, there is a sparse distribution in terms of crystal morphology, with the appearance of smaller grains and lower crystallinity, in the cable–joint transition and factory joint. These morphological variations would generate more heat during electron transport and lengthen the free paths of electrons, then lead to the occurrence of AC breakdown. The results might provide insights into the optimisation of the morphology of XLPE, in particular submarine cable factory joints.