In this paper，a cellular automaton model is proposed to describe the traffic flow at the grade roundabout crossing with the inner roundabout lane and the outer roundabout lane. Simulation results show that the traffic jam appears evidently at the intersection of the inner roundabout lane and the entering lane. The effect of the nonlinear reciprocity of the vehicle in the inner roundabout lane increases and the flux decreases with the increase of the deceleration probability. The flux of the system is improved by the increase of the turning probability.