In order deal with the attitude control of assembled spacecraft whose moment of inertia is unknown, a spacecraft combination attitude control algorithm, which is independent of accurate dynamic model, was designed based on model-free control method. Firstly, a model-free adaptive control equation was derived based on date driven control method of a single input and single output discrete-time system. Then, the model-free adaptive control method was introduced to the attitude control of the assembled spacecraft, and the model-free adaptive controller was derived. In addition, the attitude control process was optimized based on the dynamics characteristics of the combined spacecraft. Finally, the model-free adaptive control algorithm and the control process optimization algorithm were simulated under the background of the operation after target capturing. The results show that the designed model-free adaptive algorithm is feasible, and it can achieve precise control of assembled spacecraft whose inertial parameter is unknown. By optimizing the control process, fast convergence of attitude control can be achieved with a higher accuracy.