A humidity measurement method based on a fiber loop cavity ring-down spectroscopy system with low gain and low noise erbium-doped fiber amplifier is proposed and experimentally demonstrated. The influence of an erbium-doped fiber amplifier placed inside and outside the fiber ring on the pulse curve and the number of pulse peaks in the annular cavity is analyzed. A length of 2 m low-gain and low-noise erbium-doped fiber in an erbium-doped fiber amplifier is used to reduce waveform distortion and compensate for the noise attenuation of the cavity. Measurement of relation humidity is performed with the fiber loop cavity ring-down spectroscopy system, and the change of relation humidity is gained by recording the ring-down time τ of light pulse in the fiber loop. Results indicate that a good linear relationship is met between relation humidity and τ in the range of 30% to 100%. The sensitivity and accuracy of the fiber loop cavity ring-down spectroscopy system are 3.826 79 μs/RH and 0.994 77 respectively. So it can be employed for the fields of industrial detection, environmental inspection and medical diagnosis.