A new method for measuring the refractive index of micro-quantity liquid based on imaging principle of a coaxial spherical surface optical system is introduced. LED (λ=580 nm，FWHM: 32 nm) is used as light source and CCD camera as the equipment of receiving image. The required sample is less than 0.002 mL. The liquid to be measured is closed in a glass capillary, so that is benefited to measure the poisonous, volatile and absorptive liquid samples. Four liquid mediums, water, ethanol, ethylene glycol and glycerin have been measured, and the precision of obtained refractive index is 0.0001, 0.0002, 0.0003 and 0.0003, respectively. Based on an analysis on the sensitivity of refractive index and the depth of field of the measurement system, the ways to improve the measurement precision are proposed.