A novel fiber-optic humidity sensor is fabricated by using a side-polished fiber (SPF) coated with a film of reduced graphene oxide (rGO). The sensor can achieve power variation of up to 6.9 dB in the high humidity range [the relative humidity (RH) is 70% to 95% ], and display linear response with correlation coefficient of 98.2% , sensitivity of 0.31 dB/(%RH), response speed of faster than 0.13 (%RH)/s, and good repeatability in RH from 75% to 95%. Theoretical analysis of sensing mechanism can explain the experimental results, and reveal the broad applying prospect of the sensor for other kinds of chemical vapor detections. Relying on novel sensing mechanism, this graphene-based optical sensor provides a beneficial complement to graphene-based electrical chemical ones, and can promote the employment of graphene in chemical sensing techniques.