A novel humidity sensor based on open cavity Fabry-Perot (F-P) interferometer which is formed by fusion splicing a short section of single-mode fiber (SMF) between two sections of single-mode fibers with a large offset splicing method is proposed. The proposed structure can act as a nano-composite hygrometer with high sensitivity by filling the cavity with a humidity sensitive material polyacrylamide (PAM). It is realized that the humidity can be measured by monitoring the interference spectrum shift of the sensor when the refractive index of PAM changes due to absorbing water vapor in the air substantially. The experimental results show that the interference spectrum shift is about 4 nm when the surrounding humidity varies in the range of 38%~78% and the sensitivity of interference spectrum shift versus the relative humidity is about 0.1 nm/%; the interference spectrum shift can reach up to about 59 nm when the surrounding humidity varies in the range of 88%~98% and the sensitivity of interference spectrum shift versus the relative humidity is 5.868 nm/%.