Abstract: In order to measure super-high reflectivity of any sphere optical element at different angles, such as 0, 10, 15, 30 and so on, two optical resonator structures are put forward: one is straight-cavity, and the other is folded-cavity. Physical length of the ring-down cavity is defined based on the concept of light path. These two kinds of measurement methods are analyzed in detail. When spherical surface is put in the ring-down cavity, it brings length misalignment of the ring-down cavity. This varied length is derived based on the beam transmission and transformation law, which can be used as length adjustment guidance of the ring-down cavity for reflectivity measurement. In addition, according to reflectivity measuring principle of cavity ring-down, mathematical expressions of reflectivity measuring for spherical components are deduced at different angles, which are discussed separately. The results expand the super-high reflectivity measurement scope of optical elements for the cavity ring-down method.