With the development of the aerospace industry, the demand for on-orbit construction of large space platforms is increasing. However, the current on-orbit construction is in its infancy and faces the problem of error accumulation and inability to compensate, making the on-orbit construction extremely risky. To compensate for the accuracy of the on-orbit constructed truss, this project proposes a concept of a variable-geometry truss robot, which has high stiffness, large load-bearing capacity and high positioning accuracy, and can be used as a support structure and a height adjustment mechanism. In this paper, the structural design of the variable-geometry truss robot is carried out, and a modular structure composed of active nodes, passive nodes, pedestal nodes and drive rods is constructed. The forward kinematics problem of the octahedral parallel robot is deduced, and the mechanical performance analysis of the variable-geometry truss robot is carried out, including its stiffness analysis and dynamic analysis, which provides new ideas and methods for solving the problem of on-orbit construction error compensation.