An improved two-lane cellular automaton model for traffic flow was proposed to d escribe the highway traffic under the opening boundary conditions by considering the relative motion of vehicles and the relation of deceleration probability. N umerical simulation have been carried out. The results show the complicated evol ution process of traffic flow. The flow of vehicles can be controlled by the cou pling coefficient b. Different values of b have different effects on the critica l point from free to jam phase.