A highly sensitive flow rate sensor based on high-order whispering gallery mode (WGM) in an optofluidic microcavity is proposed and experimentally demonstrated. The microfluidic flow rate can be measured by detecting the shift of the resonance wavelength of the high-order WGM, which is generated by coupling the ultrafine fiber cone with the microfluidic microcavity. The detectable flow rate range is 4 to 100 μL/min and is segmented into two parts according to the sign of the resonance wavelength shift. The resonance wavelength for the flow rate of 4 to 16 μL/min is blue-shifted with a sensitivity of 48.8 pm/(μL/min), and the resonance wavelength for the flow rate of 16 to 100 μL/min is red-shifted with a sensitivity of 26.3 pm/(μL/min) with the heating temperature set to 55°C.