According to the characteristics of a new type of complex joint in the space steel structure, one model of automatic assembly system with 6-DOF was designed. Base on Denavit-Hartenberg method and matrix inverse operation principle, the positive and forward kinematics algorithm of an assembly machine is proposed and the main parameters of pose algorithm are calculated. The automatic assembly machine was designed, and its structure and principle of operation strategy was elaborated.By the control method of point to point, experimental data are obtained and then are compared with the theoretical data. The results show that this pose algorithm and its assembly control strategy is available. The maximum deviation of the rotating joint is 0.11 degrees and the maximum deviation of transfer joint is 0.1mm, which can meet the assembly demand in practice.