The BaFe0.4Sn0.6O3/BaBiO3 composite negative temperature coefficient (NTC) thermistor ceramics were prepared by conventional solid-state reaction method. The raw materials used are composed of conductive BaBiO3 phase and high resistive BaFe0.4Sn0.6O3 phase. The conductive mechanism of thermistor ceramics before and after the percolation threshold (containing 12 mol% of BaBiO3), is investigated by impedance analysis. For compositions with 5 mol%—8 mol% of BaBiO3, the contribution to the conductivity is mainly due to the grain boundary (Rb), grain (Rg), and grain shell (Rs) existing in the BaFe0.4Sn0.6O3. In the range of 10 mol%—12 mol% of BaBiO3, the grain (Rbg) and grain boundary resistance (Rbb) corresponding to the BaFe0.4Sn0.6O3 melted with BaBiO3 in composite ceramic, are also main factor governing the resistance magnitude, the values of which are lower than the other main sources like Rb, Rg, and Rs. For composition x=0.15, the values of Rbb and Rbg are higher than that of Rb, Rg and Rs. For BaBiO3 contents around 20 mol%, the resistance is mainly determined by the values of Rbb and Rbg. In addition to the electrode-specimen interface, all of the components in composite ceramic show NTC feature. The NTC composite ceramics with different BaBiO3 contents show nonideal Debye-like behavior, and the conduction mechanism of the composite ceramics is of the localizing type.