Abstract: To explore the application potential of modified polypropylene (PP) as insulation material for high-voltage cables, modified PP insulation material and the insulation layer of 110 kV three-layer co-extruded PP power cable were taken as research objects. Differential Scanning Calorimetry (DSC), Polarizing Optical Microscope (POM), X-ray Diffraction (XRD) and AC breakdown test were adopted to systematically study the intrinsic non-isothermal crystallization behavior of PP under different cooling rates, and analyze the variation rules of crystalline micromorphology, crystal parameters and electrical breakdown performance of the inner, middle and outer layers of cable insulation. The results show that the cooling rate has a significant effect on the crystallization peak temperature and crystal perfection of PP, and the crystallization peak temperature decreases with the increase of cooling rate. Affected by the coupling effect of shear flow field and radial temperature gradient during cable extrusion, the crystallinity of insulation layer presents a gradient distribution of inner layer > middle layer > outer layer. The inner layer has large and regular spherulites with fully developed crystals, while the outer layer has fine grains and numerous crystalline defects. The full width at half maximum of crystal plane diffraction peaks of each insulation layer shows obvious differences. The breakdown field strength is strongly correlated with crystallinity and spherulite morphology. The outer layer of cable insulation possesses the best breakdown performance, while the inner layer is the weakest. The research can provide experimental basis and theoretical support for extrusion process optimization, microstructure regulation and insulation performance improvement of high-voltage PP cables.