In order to keep a lightweight structure, high stiffness, strength and thermal dimensional stability, an 18-strut truss main support structure was designed for large-scale and off-axis Three-mirror Anastigmat (TMA) space camera. The merit and demerit of main support structure of off-axis TMA space camera with overall framework and truss support structure were analyzed. The truss support scheme was confirmed on the basis of optical system and the whole structure size characteristic of the camera. The dynamic and static stiffness single-layer 6-strut and double-layer 18-strut truss structure were analyzed and contrasted. The materials of truss main support structure were selected reasonably. The influence factor of the strut fundamental frequency was analyzed, and then the structure parameters of the strut were optimally designed. The mechanical and thermal characteristics of 18-strut truss support structure were analyzed. Finally, adhesive strength between strut and pipe joint was tested. As shown as the result of finite element analysis, the fundamental frequency of truss main support structure is 78.4 Hz, and the maximum tilt angle of the secondary mirror is 4.6″ around Y axis under the action of gravity in X axis direction. Dynamic and static stiffness index meet the design requirements. The tensile test results show that the strut tensile strength of the adhesive joint is 5.85 MPa, which is satisfied to strength requirements.