Abstract: With the rapid development of ultra-large-scale integrated circuits, micro-integrated high-density semiconductor components urgently require low dielectric materials. In this work, epoxy sesquisiloxane (EOVS) with cage structure was synthesized and blended with epoxy resin E51 to obtain the nanoporous EOVS/E51composite. As shown by SEM, EOVS nanoparticles are uniformly dispersed in the E51 matrix. With the increase of EOVS, the nanopores introduced by its cage structure make the free volume of EOVS/E51 composite increase and the density of polarized molecules per unit volume decrease, so that the dielectric constant decreases from 4.21 to 2.51 (1 MHz) when the EOVS content is 15wt%. However, the epoxy groups on EOVS provide new reaction sites, so the crosslink density of the EOVS/E51 composite system increases with the increase of EOVS, and the free volume of the composite decreases and the dielectric constant turns to increase when the EOVS content reaches 20wt%. In addition, the hydrophobicity of the Si—O—Si bond of EOVS enhances the moisture resistance of EOVS/E51 composites, while the thermal stability of the inorganic backbone and the nano-toughening effect lead to a significant increase in the heat and impact resistance of the composite, which is promising for applications in microelectronics.