The key factor of the sensitivity in the FBG-based pH sensor is analyzed in detail. A multi-thin-layer structure of the gel coated cover was proposed and implemented with a special process. The sensors with the coated thickness of 420 μm, 500 μm, and 580 μm were built up, respectively. The corresponding spectral shifts of 0.08 nm, 0.13 nm, and 0.22 nm were detected when the pH sensors were soaked in the pH value of 3-9. Meanwhile, the sensor with the gel layer thickness of 580 μm was measured in the optimum measurement time period with the pH value changing from 3-12, in which the detected sensitivity of 52 pm/pH was achieved in the pH range of 6-12.