This paper investigates the control of relative orbit for Rendezvous and Docking between nearby spacecraft and target spacecraft based on the sliding mode control theory. Firstly, the model of relative movement between two spacecrafts is established by using nonlinear equations. Secondly, by using the sliding mode control theory and the properties of fractional derivative, an integer order PD controller and a fractional order PDα controller are designed. The designed controllers are then applied to the perturbed and unperturbed systems. The numerical simulation results show that the sliding mode control method is effective. Moreover, comparing with PD controller, the PDα controller costs less time but more energy to achieve the same control objectives.