Humidity sensors are crucial in a multitude of sectors, including environmental monitoring, industrial processes, and life sciences. As technology advances, the demand for enhanced performance in humidity sensors has been on the rise. Compared to conventional capacitive and resistive humidity sensors, optical fiber humidity sensors have recently gained significant attention due to their compact size, swift response, robust anti-interference capabilities, extended transmission range, heightened sensitivity, and remarkable stability. Among them, Fabry-Perot microcavity sensors crafted on optical fiber facets have become a focus of research, particularly due to their straightforward fabrication and integration with humidity-responsive materials. Laser-induced waveguide self-growth technology, a method capable of fabricating beam-profile waveguides within photosensitive medium materials according to the intensity distribution of the incident beam, enables rapid preparation of sensors with polymer microcavity structures on optical fiber facets. The photosensitive polymers on the fiber facets undergo chain polymerization reactions induced by laser light, forming a highly cross-linked three-dimensional network structure. The humidity-sensing mechanism of polymer microcavity humidity sensors stems from the swelling or dehydration of the internal three-dimensional network structure upon environmental humidity changes, which leads to variations in the cavity length and refractive index of the polymer, thereby inducing spectral wavelength shifts in the interference spectra. Monitoring these wavelength shifts enables humidity sensing. This paper, embarking from the sensor's structure and principle, delves into the detailed investigation of the sensor's manufacturing process, humidity sensing performance, and respiratory monitoring. It proposes a cost-effective and rapidly fabricable humidity sensor with a polymer microcavity structure on the optical fiber facet. Leveraging laser-induced waveguide self-growth technology, the paper introduces a secondary self-growth method to achieve a higher surface-to-volume ratio and interference spectral contrast. Through precise control of laser power, exposure time, and utilizing the integrated stepper motor of a fusion splicer, a polymer microcavity with a cavity length of 47.65 μm was successfully created on the fiber facet. Changes in the cavity length and refractive index of the polymer microcavity in response to ambient humidity variations result in interference spectral shifts, enabling the accurate detection of relative humidity through spectral shift demodulation. The experimental results indicate that the sensor exhibits an interference spectral contrast of 21.6 dB and a free spectral range of 16.6 nm. It possesses an average sensitivity of 150 pm/%RH within a relative humidity range of 40%RH to 90%RH. Linear fitting of the mean wavelength shifts corresponding to different humidity points in multiple humidity rise-and-fall experiments yields a linearity of 0.985 83. During a 5-hour stability test, the maximum fluctuation in wavelength and reflected optical intensity was 0.208 nm and 0.082 dB, respectively. To further explore the sensor's practical application potential, a real-time breathing monitoring experiment lasting 3 minutes was conducted, revealing a response time of merely 0.8 s and a recovery time of 5.7 s. In summary, the proposed optical fiber humidity sensor, with its simple fabrication process, high sensitivity, rapid response, and excellent stability, shows great potential for applications in meteorological environmental monitoring and wearable healthcare fields. In the future, it is anticipated that the integration of polymers with micro- and nano-structured materials, such as graphene and MoS₂, will be employed to fabricate end-face microstructures, thereby achieving an enhancement in humidity sensitivity.