Br2 doped PANI was synthesised by chemical oxidation method and absorption of bromine vapor. MWNTs/PANI and MWNTs/Br2 doped PANI composites were prepared by mechanical blending method, The new composite exhibits as high electrical conductivity as that of pure MWNTs. 5~10 S·m^-1. Fourier transform infrared spectroscopy (FTIR), thermo gravimetric (TG), ultraviolet-visible absorption spectra (UV-Vis), X-ray powder diffraction (XRD) and X-ray spectroscopy (XPS) were used to study the electrical conductivity properties and mechanism. The results showed that π-π and p-π system is formed between MWNTs and doped PANI . MWNTs combined with the Br2 doped PANI to form charge transfer composites, then form individual conductive units, which play a critical role in forming the conducting net of composites. With the increase of conductive units, conducting system is formed by conductive unit connection and the conductivity of composites reaches the maximum.