Based upon the single-lane traffic cellular automaton (CA) model introduced by Negel and Schreckenberg, an improved single-lane traffic CA model has been proposed by the consideration of the relative motion of vehicles and the relation of deceleration probability with the density. Numerical simulations have been carried out.The results show the complicated evolution process of traffic flow.The flow of vehicles can be controlled by the definition of the relation between deceleration probability and the exponet v of density pnoise～ρv.Different values of v have different effect on the critical point from free phase to jam phase.The simulation agrees with the measurement as v is about 0.75.With the increase of vehicles and the evolution process of traffic flow,the free and jam phases will become unsteady and appear alternatively, similiar to the propagation of wave.