This paper presents a kind of relative humidity (RH) sensor,which is based on fi ber Bragg grating (FBG) coated with polyimide (PI).Substructure morphology,humid ity sensing characteristics, long-term stability and hysteresis characteristics of the humidity sensor are s tudied.The preparation and coating processes of PI are improved,and a new method to enhanc e the coupling between PI solution and optical fiber grating is proposed.Four typical surface coating structures are determined by scanning electron microscopy (SEM).Experimental results show that the PI coating thickness is less than 75μm,the fitting degree of humidity characteris tic curve is greater than 0.99,the standard deviation is less than 0.5,the fitting degree of temperat ure characteristic curve is more than 0.98,and the standard deviation is less than 0.5.The humidit y sensitivity of the sensor is 7.9pm/%RH,and the nonlinear error is 2.19% at 45μm coated thickness .Hysteresis error is 2.6%,and the relative humidity fluctuation range is less than 0.8%RH.The deve loped device can realize the temperature compensation for humidity measurement sensor.The FBG humidity sensor also has good repeatability and stability.When compared to the conventional techniques,the novel fiber optic sensor provides a robust,relativel y low-cost and continuous monitoring method.