This paper presents a new kind of all-optical hygrometer based on the sense of wet coating on the side of fiber Bragg grating(FBG).We have selected modified polyimide(PI) as humidity materials because it has features of high humidity sensitivity and good linear coefficient moisture expansion of 9.431 2×10-5%RH.And we have detected the humidity sensitivity and the response time of 5 sensors with different coating thicknesses.We find that the results of experiment match well with theoretical calculations,and the humidity sensor has the best sensitivity and response time,with the coating thickness of 21 μm.Its sensitivity is 2.67×10-6/%RH,and the response time is less than 8 s.It has good prospects for practical applications.