A chipless humidity sensor integrated with RFID is presented in the paper. The slot scatterer structure is fabricated on the copperclad FR4 by grooving etching. The encoding unit stores the ID information by the combination of the different length slots. To make a scatterer sensitive to the humidity on the premise of low cost, the silicon nanowires are deposited on the surface of sensing unit without any posttreatment process. Based on the principle of radar electromagnetic backscattering, two horn antennas with cross polarization are utilized as the transmitting and receiving antennas so that to measure the transmission coefficient curve. The experiment results show that a stable linear relationship between the center resonant frequency and the relative humidity in the frequency band of 215 MHz is appeared at the temperature of 25 ℃ and the range of 30％~90％RH, the sensitivity of the sensor exceeds 2.9 MHz/% RH.