A toughening and reinforcing cyanate ester (CE) with a lower dielectric constant (kappa) was prepared by introduction of a hybrid nanoporous silica material POSS-MPS, which with the structure of the wormhole-framework-structured mesoporous silica (MPS) covered with glycidyl polyhedral oligomeric silsesquioxane (G-POSS) by covalent bonds. POSS-MPS/CE showed better effect to decrease kappa due to more voids retained in composites. POSS-MPS exhibited a balanced effect to improve mechanical and thermal properties. The impact strength, flexural strength, and flexural modulus increased.