An optical fiber inclination measurement apparatus and a differential inclination measurement system are provided, which is related to the field of monitoring technology. The apparatus includes a supporting mechanism with a base and a base frame, a swinging mechanism with a pendulum, a cycloid, and a first reflective film is disposed on the pendulum. The measurement mechanism includes a first optical fiber, and an end surface of the first optical fiber is disposed opposite to the first reflective film. During an earthquake generating process, a landform changes and the pendulum swings, resulting in changes in displacement. The end surface of the first optical fiber together with the first reflective film form a Fabry-Perot cavity. The cavity length of the Fabry-Perot cavity changes before and after the landform deforms, and an angle of inclination is equal to a value obtained by dividing the cavity length variation of the Fabry-Perot cavity by the pendulum length.