In this study, the functionalization of a tapered optical fiber using graphene oxide is monitored in real time by a chaotic correlation fiber loop ringdown system, and hemoglobin sensing is achieved experimentally with the functionalized graphene oxide tapered optical fiber as the sensor element. The functionalization of the tapered optical fiber surface using graphene oxide can be divided into hydroxylation, silanization, and graphene-oxide deposition. The effect of transmission loss is analyzed during functionalization according to the change in the ringdown time. The functionalized tapered optical fiber is tested via scanning electron microscopy. Moreover, the sensitivity of hemoglobin sensing is evaluated using different concentrations of graphene oxide in the functionalization process. The experimental results show that the functionalized tapered optical fiber exhibits a considerable increase in sensitivity by an order of magnitude compared with the unfunctionalized tapered optical fiber. The sensitivity of the functionalized tapered optical fiber is influenced by the concentration of graphene oxide during functionalization; sensitivity increases with concentration. These results can potentially be applied in the field of biosensing.