In order to meet the requirements of high location accuracy and surface figure accuracy for space mirror,material selection,lightweight pattern and the design of support were taken into account in the design of a rectangular space mirror.A new design with local open bottom,triangular hole light-weighting structure is proposed.Mirror with the back three-point support scheme,through the rational design of flexible support structure parameters is adopted to ensure high stiffness and thermal dimensional stability.By using finite element analysis (FEA）technique,the static and dynamic stiffness as well as thermal characteristics of the mirror component are analyzed.Analysis results indicate that the surface accuracy reach to λ/10 PV，λ/50 RMS（λ=632.8 nm）under gravity and 5 ℃ uniform temperature change,the first order natural frequency of the primary mirror component is 230 Hz,which can meet the requirements of space application.