Oil identification of marine oil spill is a problem for monitoring the marine environment, which needs to be solved urgently. It is very important to distinguish different types of oil. A sensor of microfiber ring resonator (MRR) for identifying the oil species of marine oil spills is developed, and the resonant spectral characteristics of the seawater and gasoline are investigated experimentally and theoretically. The sensitivity and free spectral range of MRR for identifying the seawater and gasoline are calculated numerically according to Maxwell′s equations. A Knot-type MRR sensor is manufactured, whose ring diameter is 480.31 μm, and microfiber diameter is 4.14 μm. The experimental system is established, and the resonant spectra of seawater and 93# gasoline are obtained. Results show that the resonant wavelength shifts 2.79 nm, which is mainly consistent with theoretical calculations. The sensitivity reaches to 34.86 nm/RIU, where RIU is refractive index unit. Therefore, the MRR sensor can be used for distinguishing seawater and gasoline.