In this paper, a cellular automaton model is proposed to study the traffic of urban expressway systems with on-off-ramps and accessory roads. Three lane changing rules are defined for different road sections. Simulation results show that higher on-ramp rate easily produces traffic jams on main roads, on-ramps and their upstream sections. Higher off-ramp rate easily leads to conflict with the inflow of accessory road. The system having two lanes on main and accessory roads can alleviate the jam degree and decrease the vehicles' running time when on-ramp rates are high.