Incorporating polymeric matrix with inorganic fillers is a preferential method to improve electrical properties, while functionalization on filler surface has always been employed. In this study, two interfacial microstructures were constructed by using different functionalized polyhedral oligomeric silsesquioxane (POSS) and electrical properties of polypropylene (PP) based composites were evaluated. It is found that the electrical properties are closely related to interfacial compatibility and trap. The improved electrical breakdown strength corresponds with the composite possessing better intermolecular compatibility and deeper traps. The molecular simulation further reveals that interfacial compatibility and trap energy are affected by interaction energy and electrostatic potential between functionalized groups and PP molecular. The greater interaction energy leads to more compatible interfacial microstructure, and larger electrostatic potential leads to deeper trap energy with enhanced trapping effect, thereby restraining the charge carriers injection and migration, which synergistically leads to improved electrical breakdown strength. Contrastive interfacial traps and electrical properties of PP/POSS composites by different functionalized groups reveal the effect of interfacial compatibility. Thorough understanding of the functionalized groups and interfacial effects are beneficial for tailor-synthesizing composites with expected electrical properties.