A highly sensitive humidity sensor based on Fabry-Pérot (F-P) interferometer which is formed by hollow core fiber with low sensitivity to temperature is realized. Because the optical path difference of interferential cavity depends on the cavity length and the refractive index of the media which is filled in the F-P cavity. The F-P sensor is filled with a novel nano-composite hydrogel with a strong water capacity. It is realized that the humidity can be measured by measuring optical path difference of the sensor when the refractive index of hydrogel changes due to absorbing water vapor in the air substantially. The experimental results show that the optical path difference changes from 608.7180 μm to 604.0488 μm when the surrounding humidity varies in the range of 38%-98% and the sensitivity of optical path difference versus the relative humidity is 77.82 nm/(1%).