A method for measuring the refractive index of liquid in closed containers with an optical low coherence interference was proposed. The broadband low coherence laser diode light source and improved Michelson interferometer were used in the experiment. A container with liquid to be measured was placed in the optical path of a measuring arm. When the optical paths of the reflected light from the front and rear walls of the container were equal to the optical path of the reflected light from the reference arm respectively, interference would occur. The alternating interference signal was obtained by using a single photo-detector. By calculation, the optical path difference between the front and rear walls of the container could be obtained, and then the refractive index of the liquid to be measured would be calculated. The experimental results show that the translation accuracy of the reflector can be controlled to 5 μm within the optical delay line. The accuracy of the refractive index measurement can reach 10^-4 orders. The method can be used for real-time non-contact monitoring of liquid concentration or refractive index in biomedical, food inspection and other fields.