This work demonstrates a highly hygroscopic polymer microcavity fiber Fizeau interferometer (PMFFI) that uses a single-mode fiber (SMF) tip with very little polymer attached. The polymers with specific porous structures have good hygroscopicity which is very useful for the humidity sensing application. We utilized a general laser diode (LD) with a single wavelength which is monitoring the spectral reflections of the proposed sensor. The sensing capabilities of the developed PMFFI have been experimentally investigated. Experimental results illustrate that the laser power rises when in the ascent of humidity, besides a good linear response of sensitivity and favorable spectral property are also achieved by the proposed configuration.