A racetrack micro-ring sensor with feedback loop was proposed,which make up of a U feedback waveguide,a racetrack cavity and a sensitive ring. The normalized transmission formula of the sensor was derived by Coupled-Mode theory.The effects of the transmission loss factor,the coupling factor and sensitive ring size of sensor on the normalized optical intensity were numerically analyzed. The results show that the normalized optical intensity is very sensitive to the transmission loss factor of the sensitive ring. As long as the transmission loss factor changes tinily,the normalized optical intensity changes greatly. When the sensitive ring works on the condition of weak coupling,the sensor may obtain high sensitivity. And the size of sensitive ring has great influence on sensing sensitivity,adjusting the size of sensitive ring properly,the sharp normalized optical spectra were got. Based on the results,the optimized structural parameters were given. The optimization of micro-ring structure can not only output sharp steep asymmetric Fano resonance spectral lines,but also obtain a high Fano resonance spectral slope,which was better to enhance the sensor sensitivity. For a measurement system of 30 dB signal-to-noise ratio,a detection limit can reach to 4.48×10-8 refractive index units in the optimized feedback racetrack micro-ring sensor.