In order to reduce the impact of eye’s self-control on the diopter measuring, an open-view optical system of natural refractor is established and its optical imaging characteristics and diopter measuring method are investigated. And a method of optometry by measuring the ring thick of the reflection image, rings thick method for short, is proposed. Firstly, the optical path of the open windows type natural refractor and its working principle are introduced. Next, the relationship between diopter and far point distance is obtained through theoretical analysis and the ring thick method is proposed to measure the far point distant. Then, build related eye models to verify the optical system’s simulation performance by Zemax. Finally, set up the related optical system to test nine simulation eyes. The experimental results indicate that the relative error of the nine simulation eyes is controlled within 7%. These results satisfy the requirement of diopter measuring very well and the optical system is characterized by compact structure, low cost, good accuracy, and high stability.