In order to achieve high-sensitivity monitoring of chemical materials and drugs with low refractive index, a new type of high-sensitivity photonic crystal fiber (PCF) sensor with elliptical side core is designed based on the surface plasmon resonance (SPR). A circular hole and three different sizes of elliptical holes constitute the sensor air hole, and the gold nano film is coated in the left elliptical hole. When light enters the sensor, it will generate resonance at the gold thin film. The refractive index of the liquid to be measured can be obtained by analyzing the spectrum obtained from this physical phenomenon. The finite element method is adopted to systematically investigate the effects of ellipticity of elliptical holes and the thickness of gold thin film on the sensitivity of the proposed sensor. The results reveal that the maximum sensitivity of the sensor reaches 95 000 nm/RIU (RIU is refractive index unit) when the refractive index of the liquid to be measured ranges from 1.41 to 1.44. In view of the existing researches, the sensitivity is 3.7—47.5 times of that of typical available PCF-SPR sensors within the same range of refractive index. Therefore, the proposed PCF-SPR sensor can be widely applied in fields, such as material monitoring, food safety, and biomedicine.