In this paper, a new method for measuring liquid refractive index based on a spherical aberration-free liquid-core cylindrical lens is presented by analyzing an experimental defocused image on the focal plane. A spherical aberration-free liquid-core cylindrical lens is designed and fabricated which has small spherical aberration and accurate imaging in a wide range of refractive index. Using the designed lens, according to the relationship between the experimental image and the liquid refractive index, only one defocused image is needed to measure the liquid refractive index. The method is used to measure the refractive indexes of 21 groups mixed solution at room temperature (20℃). The measured results are in well agreement with those using Abbe refractometer. The defocused images simulated by ZEMAX and ray tracing method are consistent with the experimental results. The experimental and simulation results show that the method has the characteristics of simple system, good stability, simple operation and fast measurement speed.