In this work, the effect of the polar groups modified polyimide fiber Bragg grating relative humidity sensor was studied. Polyimide films containing different polar groups were deposited on fiber Bragg gratings by an impregnating method to form different relative humidity (RH) sensors. The experimental results show that the addition of a carboxyl group and a hydroxyl group in the synthesis could improve the humidity sensitivity (2.28 and 1.59 times, respectively) from 35% RH-95% RH. The sensor based on modified polyimide still has good linear response to humidity and temperature. In addition, the sensors containing a carboxyl group and a hydroxyl group can shorten the response time of the humidity sensor and improve the stability of the sensor at the same time.