In this paper, an inclinometer based on a vertical pendulum and a fiber Bragg grating Fabry–Perot cavity (FBG-FP) is proposed. A low-damping rotation structure is used to reduce the mechanical frictions of the pendulum system and induce a wavelength shift of FBG-FPs. We find that the sensitivity can be maximized by optimizing the parameters of the inclinometer. Using a high-resolution demodulation system, a sensitivity of 179.9 pm/(°), and a resolution better than 0.02″ can be achieved. Experiments also show that the proposed inclinometer has good linearity and repeatability.