In the present study, fast response resistive-type PI/multiwall carbon nanotube (MWNT) composite films were demonstrated. A composite film with a loading of 3 wt% MWNTs (PIC30) possesses a very linear response nature, a linearity correlation (R2) of 0.99157 and a sensitivity of 0.00146/%RH. The response time were less than five seconds and the resistance changed synchronously with different humidities. The recoverable and repeatable resistive responses affirmed the high efficiency of this film for fast humidity detection. A negative temperature effect was found and proper temperature compensation should be considered in the future applications. The surfaces of the films were found as an organized structure with nano-size dimples, which is helpful for absorption of water molecules. The proposed sensing mechanisms are discussed and it related with tunnel effects, doping of MWNTs by water and a barrier effect between MWNTs.