A humidity sensor with a multimode fiber-air cavity-multimode fiber-polyvinyl alcohol (PVA) film structure based on Fabry-Perot interference is proposed. The influence of the curvature of air cavity reflecting surface on the light propagation and reflection spectrum is studied, and a method to increase the radius of reflecting surface by arc discharge is proposed. The experimental results show that the proposed method reduces the interference loss of light in the fiber core, allows more light to participate in interference, and improves the contrast and fineness of interference fringes. A layer of PVA film with a thickness of 14 μm is coated on the optimized multi-cavity end surface. The average sensitivity of the sensor can reach 73.24 pm/% when the relative humidity of the environment is 29.1%--81.8%, and has good time stability.