Negative permittivity (ε′ < 0) has been a vital and eye-catching electromagnetic parameter for designing new-generation electrical devices. However, ε′-negative materials generally suffer from the overhigh absolute value of ε′ and serious frequency dispersion, especially in the radio-frequency region. Hence, carbon nanotubes/polystyrene (CNTs/PS) membranous composites were ingeniously designed and fabricated by a spin-coating procedure. The ultraweakly and frequency-stable negative permittivity (ɛ′ ~ -200) was amazingly achieved over 10 kHz-1 MHz region, attributing to the weakly low-frequency plasmonic state within a 3-dimensional (3D) CNTs network. With increased CNTs content, hopping conduction behavior in composites evolved to metal-like conduction. The impedance analysis based on equivalent circuit models confirmed the inductive characteristic of ε′-negative materials. The obtained flexible CNTs/PS membranous composites could significantly enrich their application on novel wearable electrical devices.