Humidity measurement has been of extreme importance in both conventional environment monitoring and emerging digital health management. State-of-the-art flexible humidity sensors with combined structures, however, lack sensing reliability when they subject to high humidity with condensation and/or liquid water invasion. Here, we report a free-standing humidity sensor by creating a stable open porous graphite structure and controlling the number of oxygen-related groups at the molecular level. The sensor indicates high sensing reliability against water liquid wetting and machine washing as well as many mechanical deformations. We demonstrate the sensor has wide potential applications in challenge environmental monitoring and wearable body area sensing networks. Particularly, the concept of the humidity sensing strategy is applicative to not only cellulose-based materials such as cotton and linen, but also protein-based materials such as silk, paving a new route for producing high-performance and cost-effective humidity sensors.