One of the most important factors for influencing interferometer measurement accuracy is coherent noise recently. To improve measurement accuracy, the coherent noise of interferometric system should be reduced effectively. Ring light source as an effective means to suppress the coherent noise has been widely used in engineering practice. In this paper, with different ring radius, the interference measurement accuracy is analyzed based on interferometer model. And the influence of interference measurement results increase in a certain ring radius, along with the increase of interference cavity length. Simulation results show that, with the increase of the ring light source ring radius, the system error of measurement results increase with the increase of the ring radius, in the same ring radius, with cavity length be longer, the system error of measurement is greater.