In this work, a low-humidity sensor based on Au-HKUST-1 was proposed by the method of post-synthesis modification. The hydrophilicity and conductivity of the sensing film were improved by the catalysis of Au nanoparticles (Au NPs). The Au-HKUST-1 sensor performs high sensitivity in low-humidity environments, with a response value of 30.4 within the 10%–16% relative humidity (RH) range (1 kHz). Meanwhile, the sensor exhibits short response/recovery time (29/39 s) and high resolution (1% RH), as well as good repeatability. Furthermore, the sensing film could be used for humidity monitoring by colorimetric.