Carbon nanotube (CNT) was modified with TDI molecules in order to improve the dispersion of carbon nanotube fillers in organic phase. Semi-conductive layer prepared by epoxy matrix with 0.045 phr CNT was second-time bonded to the neat epoxy for preparing double-layer composite. Broadband dielectric spectroscopy was performed to measure the complex permittivity of neat epoxy, epoxy — CNT/epoxy double-layer and CNT/epoxy composites. Compared with neat epoxy, the addition of CNT fillers makes bigger dielectric constant, the double-layer composite has the biggest dielectric constant. The space charge injection in the epoxy resin insulation layer was measured by pulsed electro-acoustic (PEA) method. The charges accumulated at the interface between insulation layer and semi-conductive layer are homo-polar as the charge close to the anode, which mainly from the injection of electrode at low temperature, but mainly related to the interfacial polarization at high temperature. The semi-conductive layer has certain effect on hinder the charge injection from anode to the insulation layer. And the semi-conductive layer is helpful to the dissipation of space charge after depolarization, especially at high temperature and under high applied electric field.