Abstract: The bisphenol A type cyanate resin (CE) was modified with diglycidyloxypropyloctaphenyl double-decker silsesquioxane (EP-DDSQ) as a modifier to prepare the EP-DDSQ/CE composite. The results show that EP-DDSQ accelerates the curing reaction rate of EP-DDSQ/CE composites. When the mass fraction of EP-DDSQ is 1wt%, the impact strength, flexural strength and flexural modulus of EP-DDSQ/CE composites are 16.9 kJ/m², 123.6 MPa and 3.37 GPa, which are 80.5%, 21.6% and 14.0% higher than the pure CE, respectively, indicating that the proper amount of EP-DDSQ can simultaneously improve the toughness and strength of EP-DDSQ/CE composites. The thermogravimetric and dynamic mechanical analysis results show that the glass transition temperature, initial decomposition temperature and residual mass of the EP-DDSQ/CE composites increases with the addition of appropriate amount of EP-DDSQ, and the maximum decomposition temperature remains basically unchanged. The dielectric performance test results show that the dielectric constant and dielectric loss of EP-DDSQ/CE composites have a tendency to decrease, indicating that the addition of EP-DDSQ gives the EP-DDSQ/CE composites better dielectric properties.