The nonlinear sensing property and electric mechanism of carbon humidity-sensitive membranes under investigation were manufactured by using hydroxyethyl cellulose （HEC）, carbon black （CB） and humidizer sorbitol. Microstructures examined by scanning electronic microscopy （SEM） show that, an effective carbon black electric network in humidity-sensing membrane would make the membrane’s resistance to have strong nonlinear property near 80%RH; so that the amount of 2wt% carbon black is in the proximity of the electric percolation threshold. The voltage-current characteristic suggested that the nonlinear sensing property and electric mechanism of the carbon humidity membranes are closely related. Particularly, impedance spectroscopy of sample at 92%RH was made up by two semi-circular arcs and a straight tail near 45° angle. The membrane′s equivalent circuit was three RC circuits in series that corresponds to the contributions of the carbon bulk resistance, CB-grain-boundary resistance and electrode contact resistance, respectively.