The large electrically stimulated actuation in high-performance actuator materials requires a careful investigation of how the variation in strain would affect the mechanism of inside polarization and thus the final dielectric properties of the carbon nanotube electroactive composites. Herein, we systematically studied the tensile strain's effect on the dielectric properties of multiwall carbon nanotube (MWNT)/poly(vinylidene fluoride) (PVDF) nanocomposites, by quantitatively characterizing the inside microstructure shift during uniaxial mechanical stretching. The results showed that MWNTs became aligned in response to the external stretching and the preferential alignment degree increased with the tensile strain. This in turn triggered a complex changing law of macroscopic dielectric properties. Maximum increment in the dielectric permittivity of the composite can reach 30% at 102 Hz relative to that before stretching. Evolution of conductive pathways and formation of microcapacitors during stretching can be invoked as being responsible for the large variation of dielectric properties. This was finally confirmed by using an equivalent circuit model of two parallel RC circuits in series to analyze the impedance data.