Abstract: The nano SiO₂/low density polyethylene(LDPE) composites were deep mixing by a parallel twin-screw extruder, the effect of the process on the dispersion and direct current breakdown strength and space charge characteristics of nano SiO₂ particles in nano SiO₂/LDPE composite system were investigated by SEM, direct current breakdown strength test and variable temperature space charge test, comprehensively evaluated dispersion improvement and the effect of the mechanical shear degradation on the electrical properties of nano SiO₂/LDPE composites in molten state. The results show that with the increase of mixing times, nano SiO₂ particles are dispersed more uniformly in the LDPE; the direct current breakdown strength of deep mixing increases to 433.1 kV/mm at room temperature compared with single mixing; With the increase of mixing times, the ability to restrain the space charge becomes stronger at low temperature, but the inhibition ability becomes worse at high temperature above 60℃. The increase of mixing times improves the dispersion of nano SiO₂ particles and increases the interface with the LDPE matrix, meanwhile, the nano SiO₂ particles also increase the thickness of the lamellae of the composites, increase the crystallinity, increase the interface area and mechanical properties with the improvement of the dispersibility, both of them promote the improvement of electrical properties. However due to the degradation of material caused by deep mixing, the increase of structural defects affects the space charge suppression performance in the high temperature region of nano SiO₂/LDPE composites.