Relative Humidity (RH) is a physical parameter which reflects the degree of atmospheric dryness. It has wide applications in agriculture, biology, petrochemical fields, food-processing, medical treatment and Internet of Things (IOT) technology. Fiber-optic humidity sensors have attracted widespread attention from scholars due to their high-measurement accuracy, anti-electromagnetic interference, multiplexing, and distributed sensing. Especially, humidity sensors based on Fabry-Pérot Interferometers (FPIs) have been widely valued for their high repeatability, compact size and high-sensitivity. Various humidity sensors based on silica fiber-optic have been reported. In the majority of fiber-optic humidity sensors, however, the sensitivity of RH detection can be improved by coating hygroscopic materials. At the same time, because they are fragile and inflexible, silica optical fibers must be treated carefully. This increases the cost and complexity of the fabrication process. Therefore, it has great research significance and application value to study new materials and structures. In order to simplify the manufacture of sensor and obtain excellent humidity sensitivity, a composite humidity sensor based on Polymethyl Methacrylate (PMMA)-microsphere and Single-mode Fiber (SMF) is designed and fabricated. Since the Polymer Optical Fiber (POF) material is very easily heated to form a molten state. In this work, the proposed sensor can be fabricated with an electric soldering iron. When the soldering iron is heated to about 70℃, place it 1 cm below the POF to slightly attach the POF to the SMF. Slowly rotate the heating source (i. e., electric iron) to allow the POF to be evenly heated. A Fabry-Pérot (F-P) cavity is formed between PMMA-microsphere and the fiber endface. It should be noted that adjusting the heat source temperature and distance between fiber-optic and heat source, the temperature of heating can be controlled in the heating process. Compared with the alcohol lamp heating method, the electric soldering iron heating method can provide a stable heating source and high safety factor in the experiment. And then, the humidity sensing characteristics of the sensor are theoretically and experimentally studied. When the external ambient humidity rises, the volume of PMMA-microsphere can expand after absorbing moisture from the surrounding. It causes the length of the F-P microcavity to grow. Then, the peak (or valley) of the F-P reflection spectrum shift toward longer wavelengths (i.e., red-shift). Thus, the humidity sensing can be realized. To investigate the sensing performance of the designed sensor, a test system that includes a demodulator, a humidity box, and a personal computer is built. Firstly, humidity test experiments are performed when the RH increases from 30% to 80% at a step of 10%. The humidity experimental results show that the wavelength shifts approximately linearly with the humidity changing, and the linearity reaches 0.992 26. The sensitivity of the sensor is up to 173.36 pm/%RH in the humidity range of 30%~80%. Meanwhile, the sensor is placed in the temperature box (NBD-M1200-10IC, NOBODY. China). The reflection spectra of the sensor with different temperature are acquired by a spectral acquisition with the range of 1 520~1 580 nm. Experimental results demonstrate that the interference spectrum has a red-shifted about 3.15 nm in the temperature range from 35℃ to 65 ℃. The temperature sensitivity of the sensor is 105.07 pm/℃, and the linearity is 0.989 55. In the subsequent studies, we will consider cascading FBG in the proposed sensor to solve the problem that cross-sensitivity of temperature and humidity. Finally, the proposed sensor performs well, showing a superior stability and repeatability over the test cycles in the performance evaluation actualized. When relative humidity is 33%RH and 38%RH, the resonant wavelength of the reflection spectrum has a very small shift within 60 min. The maximum deviations are 0.08 nm and 0.09 nm, respectively. The results indicate that the proposed sensor can maintain good stability in a long-term working condition. In addition, the wavelength drift deviation of the three repeated experiments is small, and the sensitivities are 176.05 pm/%RH, 170.35 pm/%RH and 173.68 pm/%RH, respectively. The average humidity sensitivity of the three groups tests is 173.36 pm/%RH. The designed humidity sensor offers numerous advantages such as low cost, high-sensitivity, simple structure and easy fabrication, which has a wide application prospect in the field of biochemical, agriculture and environmental monitoring.