In order to satisfy the requirements of high stiffness,high strength and high thermal stability of support structure between primary mirror and secondary mirror,some problems about truss design such as materials selection,structural configuration,connection technology and parameter optimization are taken into account.As an example,a 7 m focal length and a relative aperture 1∶6，space camera with Cassegrain optical system has been developed.A 3-layer 27-strut made of carbon fiber composite is used in the main support structure between primary mirror and secondary mirror in the space camera,and the design is optimized by finite element method.The analysis results indicate that the weight of the support has been reduced by 38 kilograms,and the first-order eigen frequency is 80 Hz,the angle change between primary mirror and secondary mirror under radial self-weight and 5 ℃ uniform temperature rise loading is less than 4.6″.These researches verify that the support structure is rational and feasible and can meet the requirements of the design specifications.