In this paper, (K1/4Ta3/4)(x)Sn1-xO2 (KTSO, with x = 0, 0.01, 0.02, and 0.05) powders were prepared by solid-state reaction method and used as materials for humidity sensing. The method of saturated salt solution is used to study the sensing performances of the KTSO sensors in the relative humidity (RH) range of 12%-94%. The results show that the sensor based on x = 0.05 sample with the thickness is 62.41 mu m and shows the best performances, including a high humidity response of 74.39 pF/%RH, fast recovery time of 5 s (within 5 min), good stability and repeatability. In addition, we found that the humidity response can be easily adjusted by a small dc bias. Oxygen vacancy and ion defects are believed to be the key factors of humidity sensing effect related to doping level; a series of structural coefficients, such as crystallite size, specific surface area, and micro-strain stress, are argued to the factors of the effect related to thickness.