The fiber Bragg grating (FBG) humidity sensor using polyimide as the m oisture sensitive material was studied and used to monitor the internal humidity in real time during the shutdo wn of the proton exchange membrane fuel cell (PEMFC).The sensor utilizes the effect of wet expansion of polyimide to make the axial pitch of FBG change with the change of external humidity.The method of corroding the optical fiber cladding is used to reduce the diameter of the optical fiber cladding.The experimental results show that t his method can increase the change of the relative humidity of the external environment without affecting the sensi tivity of the FBG relative humidity sensor.Speed of response.The developed FBG relative humidity sensor has a sens itivity of 3.8pm/RH,a linearity of 99.85%,and a maximum return error of no more than 9pm.When the environmenta l relative humidity changes from 50% to 85%,it is corroded the relative FBG relative humidity response time is only 15.4s.The FBG humidity sensor is placed inside the PEMFC for real-time monitoring of humidity during s hutdown.The experimental results show that the FBG humidity sensor produced can accurately reflect the changes in internal humidity during the shutdown and purge of the PEMFC.