Based on optofluidics and whispering gallery mode (WGM) theory, here an optofluidic refractive index sensor with microtube-coupled suspended core fiber (SCF) is proposed. It solves the issues of general sensors with microcavity-coupled fiber taper such as too fragile, unstable performance due to open coupling, poor portability and repeatability, while overcoming the poor performance of low refractive index sensing in general full-package fiber sensors. The sensor only needs a very small amount of liquid sample (about 1.8 nL). The proposed sensor combines the excellent performance of full package, optofluidics and WGM resonator. The resonant characteristics and sensing performance of the sensor are analyzed and discussed by the theoretical simulation. The simulation results indicate that the sensor has a wide refractive index sensing range (1.330—1.700) and good performance. The resonance wavelength shift has a good linear relationship with the liquid refractive index variation. In the low refractive index region, the sensitivity is 222.5—247.5 nm/RIU, Q-factor is 1.03×103 and the detection limit is 3.64×10-4 RIU. In the medium and high refractive index regions, the sensitivity is 564.4—846.2 nm/RIU, Q-factor is up to 8.62×104, and the detection limit can be as low as 1.29×10-6 RIU. The sensor exhibits a high sensitivity, a high Q-factor and a very low detection limit.