The optical fiber humidity sensor is based on the principle of the absorption spectrum of unclad optical fiber coated with colorimetric reagent film. The absorption spectrum changes with relative humidity. The output characteristic and sensitivity of the sensor are discussed. Theoretical results show that the sensitivity of the sensor is related to the diameter and the length of the unclad optical fiber, the thickness and the absorption coefficient of the colorimetric reagent film. A new scheme of the optical fiber humidity sensing is put forward. Tests on experimental models have shown performance coinciding with theory, the measurement upper limit of relative humidity to 96%RH, the sensitivity of 0.0196dB/RH to 0.0342dB/RH and the response time of 2.5 sec.. The sensor is more compact and cheaper.