Surface flashover is the primary limitation to the development of power system and the increase of voltage level. Previous work has proved that the trap can greatly influence flashover performances, but the relationship between trap parameters and surface flashover voltage is not clear. In the paper, we study the effects of crystallization behavior, microstructure, and trap parameters on DC surface flashover performance of semi-crystallinity polymer through adding phenolphthalein which is regarded as nucleating agent in low density polyethylene (LDPE). Micro-IR spectroscopy result proves that phenolphthalein exactly exists in LDPE/phenolphthalein composite. Differential scanning calorimeter (DSC) and scanning electron microscope (SEM) are used to investigate the effect of nucleating agent (phenolphthalein) on crystallinity behavior and microstructure of LDPE, and their results indicate that the phenolphthalein modification greatly changes the crystallization behavior of LDPE. The SEM results show that the spherulite size of LDPE decreases and is distributed more uniformly with the increase of phenolphthalein concentration. The DSC results show that the crystallinity and lamella thickness increase. Thermally stimulated depolarization current (TSDC) is used to characterize the trap parameters of LDPE/phenolphthalein composites. The TSDC results indicate that the shallow trap level (γ peak) increases from 0.19 eV to 0.65 eV and the deep trap (α peak) increases from 0.81 eV to 0.99 eV with the increase of phenolphthalein concentration. Relationship between microstructure and trap parameters shows that the smaller spherulite size indicates the deeper trap level (for LDPE, the trap level increases from 0.81 eV to 0.99 eV when the spherulite size decreases from 23.2 μm to 14.9 μm), and larger crystallinity means smaller trap density (for LDPE, the trap density decreases from 1404 pC to 612 pC when the crystallinity increases from 34.51% to 43.25%). The DC surface flashover performance increases with the increase of phenolphthalein concentration, and reaches a highest value: when the concentration is 1 wt%, the highest value is increased by 48.42%. Finally, it is concluded that the microstructure of semi-crystallinity polymerinfluences the trap parameters, which affects the surface flashover performance through affecting the carrier transport process in the development process of surface flashover. The trap level and trap density play complementary, cooperation and mutual transformation roles in improving the surface flashover performances as indicated by the analysis of the " U-shaped” relationship between trap parameters and flashover voltage.