The traffic flow at the roundabout crossing with the inner roundabout lane and the outer roundabout lane is described by a cellular automaton traffic flow model with open boundary condition. Simulation results show that the boundary condition and bottlenecks affect the traffic flow at the roundabout crossing. The enter probability α and the out probability β can change the systemic flux in a certain range. Three phases of the system are differentiated by the relationship between the systemic flux and the enter probability α,the out probability β. The systemic phase diagram is also studied. The research can provide a reference for the realistic traffic control.