A Fabry-Pérot fiber-optic humidity sensor with porous silicon film as sensitive element is proposed. The volume of liquid water in porous silicon film tends to dynamic equilibrium due to capillary effect. Water content in porous silicon film increases with the increase of the relative humidity, which results in the change of effective refractive index of porous silicon, and therefore shifts the reflected spectrum. Based on theoretical study, the experimental results demonstrate the feasibility of use of porous silicon thin film as humidity sensor. The characteristic wavelength shifts 9.6 nm as the relative humidity changes from 11% to 97%. By data fitting of the experimental results; the effective refractive index changes under different humidity conditions can be reckoned. A red-shift of interference spectrum fringe is monitored, which is consistent with the theoretical calculation.