We primarily address the dynamic humidity sensing requirements within hydrogen fuel cell test stacks. Dynamic humidity testing environments impose stringent demands on sensor stability, detection accuracy, and response speed. However, the dense structure between polyimide layers and the higher film thickness have historically led to extended response time. A fabrication approach for polyimide fiber Bragg grating humidity sensors is introduced, which involves a combination of ethanol and potassium acetate. This development results in a humidity sensor in which the treated coating accelerates the exchange of water molecules within the film’s interior. Furthermore, the moisture-sensitive inorganic salt, potassium acetate, enhances the overall water absorption rate, amplifying the center wavelength humidity sensing drift in the grating. Experimental validation confirms that the sensor exhibits high sensitivity and rapid response capabilities under dynamic humidity conditions, showcasing its potential applications in practical engineering scenarios.