Effects of the radial modes of electric field in micrtube on detection sensitivity of whispering gallery mode (WGM) based opto-fluidic biosensor are theoretically analyzed. By increasing the incident angle of the prism, different radial modes are excited in experiment through the prism coupling method. The sensitivity of the sensor with different radial modes is measured by sensing experiments using ethanol solution of different mass fractions. The results show that sensitivity is obviously affected by radial modes. The sensitivity is enhanced from 2.725 nm/RIU (refractive index unit) to 24.464 nm/RIU when the radial mode increases from 28 to 38, which is consistent with the theoretical analysis.