A new optical fiber humidity sensor with high sensitivity is reported. We effectively control the light-intensity changes in a smaller sensing area and achieve a significant increase in sensitivity by adjusting the depth of the evanescent field of the tapered fiber. The sensor is designed with an 8 µm diameter single-mode tapered fiber structure coated with a thickness of a 10 mm length carbomer layer in the tapered area. The average and maximum relative humidity (RH) sensitivities are 2.59 dB/%RH and 5.43 dB/%RH in the range of 68%-90%. To our best knowledge, the sensitivity of the sensor is highest compared with that of the previously reported. Moreover, the fast response time and recovery time of the sensor are ideal. In addition, the proposed humidity sensor has good repeatability and lower-temperature cross talk. Due to the excellent indicators, the proposed sensor has promising potential for highly sensitive RH sensing applications, especially early warning of special environments.