In this paper, urban traffic accessibility is regarded as the external manifestation of the dynamic process of the coupling of several traffic systems. Firstly, the relationship between a subway system, a bus rapid transit system, and a common bus system are analyzed, as well as the relationship between the traffic networks and the ground road network, and the coupling networks are constructed to objectively and comprehensively reflect the coordination structure of a city's traffic system. Then based on the static structure of the coupling traffic networks, dynamic process of vehicles and personnel flow is analyzed, and the networks' dynamic characteristics is studied to get a congestion propagation model, so as to provide the basis for quantitative evaluation for the accessibility of the multi-traffic system. Finally, an algorithm to solve the accessibility of the system is designed and the practical reachability of the traffic networks of a sample city is calculated.