Abstract: The refractive index is an important property of liquid，which is often detected by the optical interference method for transparent liquid. Most methods need to measure the movement of interference fringes，but it is difficult to judge whether it is exactly the integer order fringes movement，which will bring some errors. To solve this problem，this paper proposed a ring interference light path，which realized the interference spot with zero optical path difference and zero angle difference between clockwise and counterclockwise light paths，instead of the conventional interference fringes. The detection principle suitable for this optical path was also designed. After rotating the liquid box at a certain angle，the liquid refractive index can be calculated by measuring the movement of the interference spot. The experimental results showed that the relative error between the measured refractive index of deionized water and the standard value was small. It demonstrates that the design of the experimental optical path and principle are reasonable and reliable，which provides a new method for measuring the refractive index of the liquid.

Key words: optical loop interference, interfering spots, refractive index