Abstract: Polypropylene for HVDC cable insulation has good electrical properties and thermal stability but has poor mechanical strength and needs to be modified to overcome its shortcomings. In this paper, two types of crystallization modulation by blending modification and heat treatment are investigated. Elastomer with different mass fractions is first blended in polypropylene. On this basis, the blends are heat-treated to control the cooling rate during the non-isothermal crystallization process, and polypropylene/ elastomer blends with different crystalline morphologies are obtained. The relationship between the elastomer content, the cooling rate, and the crystalline morphology is analyzed based on the results of physical and electrical property tests. The influence mechanism and modification effect of crystallization adjustment on the mechanical properties and DC breakdown strength are investigated. The experimental results show that at elastomer contents of 10%, 20% and 30%, the nucleation mode changes during crystallization as affected by the cooling rate, which in turn affects the insulation properties. When the elastomer content is 40%, the effect of the cooling rate on the nucleation mode is weakened, while the small and dense spherical crystal structure enables the blends to obtain better mechanical properties and higher DC breakdown field strength, which reaches 160 kV/mm at all cooling rates. In summary, difference in crystalline morphology due to different nucleation modes is key to the performance of polypropylene/elastomer blends. The research on crystallization adjustment mechanism and performance of polypropylene/elastomer blends is significant for the development of polypropylene-based materials for HVDC cable insulation.