This study proposes a fiber Bragg grating (FBG) humidity sensor using polyimide (PI) as a humidity-sensitive material. First, to homogenously coat the FBG’s surface, a PI solution is perfused into a PI capillary. Then, a coupling agent is used to strengthen the bond between the humidity-sensitive material and FBG. Finally, three groups of humidity sensors with different thicknesses and lengths are fabricated, and their humidity sensing and temperature cross-sensitivity characteristics are tested. The experimental results show that the thicker the PI film is, the lower the fitting degree is and the higher the humidity sensor’s sensitivity is. The FBG humidity sensor with a film thickness of 0.13 mm shows a sensitivity of 5.5 pm/%RH, a detection range of 30%RH-98%RH, a fitting degree of 0.99, and a nonlinear error of 2%, with good operability, popularization, and applicability.