A simple chemical method was employed to form Fe2+ doped polypyrrole (PPY) by the introduction of hydrated ferric chloride (FeCl3·6H2O) during the polymerization of pyrrole, where FeCl3·6H2O played a role of oxidant for the polymerization and the in situ forming reduced product (FeCl2·2H2O) was well doped in PPY. The resultant Fe2+ doped PPY species were characterized by using various techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field emission scanning electronic microscope (FE-SEM). Humidity-sensitive properties of the samples were also examined. The Fe2+ doped PPY exhibited a fast response to humidity change (about 20s), in which great changes of more than three orders (nearly four) of magnitude in impedance was observed when relative humidity was varied over the whole range. The improved properties may be related to the presence of the hydrophilic Fe2+, and a possible mechanism was also provided.