A crosslinked polyelectrolyte was synthesized by reaction of lithium benzene-1,3,5-tris(olate) and terephthalic aldehyde using a one-step hydrothermal process. The structure and morphology of the resultant polymer poly(lithium benzene-1,3,5-tris(olate)) (PLBTO) were investigated by Fourier transform infrared spectrum (FTIR), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A humidity sensor based on PLBTO was fabricated and humidity sensitive properties were explored. The impedance of PLBTO sensor changed four orders of magnitude with a good linearity over a relative humidity (RH) range from 33% to 95%. The conductive mechanism of the sensor was discussed based on complex impedance plots. The results indicate the quantity of adsorption water molecules would determine the category of conductive particles, and cause the impedance change under different RH. Moreover, the PLBTO sensor showed little hysteresis, rapid response/recovery and good long-term stability, which have benefited from the crosslinked structure of PLBTO.