To develop a Φ510 mm SiC ultra-lightweight mirror, this paper proposes an ultra-lightweight structure with a section in ship form, an internal structure in regular-triangle, and a face plate reinforced with regular-hexagon. When the design weight is 1.85 kg, the structure performance is compared with those of other two lightweight structures with the same weight by adjusting all structure parameters. It is shown that in the same 6 point supportings on mirror backs, this ultra-lightweight structure owns absolute advantages in the gravitational deformation of mirror surface (RMS) as comparing with other two structures when the optical axis is pointed to the zenith. Furthermore, Patran＼Nastran finite element software is used to simulate and calculate the gravitational deformation of the mirror surface when the axis is pointed to the horizon, and its thermodynamic and dynamical characteristics are analyzed. The results present that all performance indexes of this ultra-lightweight mirror meet requirements of applications. Finally, according to the design, a mirror blank with a weight about 2.2 kg and surface density about 10.8 kg/m2 is fabricated. With excellent perforultra-lightweight mirror; SiC mirror; reaction bonding; Patran＼Nastranmance, it has been up to the today's advanced level in the SiC ultra-lightweight mirror field.