The complexity science, as a new crossing discipline, have infiltrated many areas of traffic study. In recent years, studing the problem of urban traffic system with complex network theory offers a new perspective to investigate the generation and propagation of traffic congestion. This paper studies the impact of the network topology on traffic congestion by the improved mesoscopic traffic flow model, and analyzes properties of the occurrence of traffic congestion, which can raise the effective elimination of traffic congestion control strategy as well as make reasonable transportation planning.