After analysis the collecting methods of near backscatter light in laser fusion, a metal ellipsoidal mirror was proposed to collect the near backscatter light of SG-Ⅲ laser facility. Then, the method to determine the parameters of ellipsoidal mirror was discussed. The mirror is composed of four pieces, which makes it entering into the target chamber easily. The mirror is made of super hard aluminum, and a triangular weight reducing slot is designed on the back of the mirror to reduce its weight and improve its mechanical stability. The overall structure of the optical system was introduced, and the light gathering performance was simulated using optical software. The key issues in ellipsoidal mirror design, including material selection and structure selection, were analyzed. In order to avoid rework, a semiconductor laser array was developed specially to test the surface in design processing. The stray lights in the system were analyzed, physical isolation, optical absorbing and optical filters were used to control them. The ring ellipsoidal mirror with a weight of 48 kg has an outside diameter of 1 200 mm, an inside diameter of 400 mm, the maximum off-axis magnitude of 1 411 mm, and the maximum thickness of 67 mm. The image spot size is about 1.2 mm within a field of view 5 mm. This system mentioned above can offer a reference for near backscatter diagnostic in inertial confinement fusion research.