The urban transportation system is one of the lifeline systems for earthquake relief. At present, most of the studies on the post-disaster connectivity assessment of transportation networks mainly focus on the surface road transportation network. With the development of urban underground space and the increasing mileage of urban subway network, urban surface and underground transportation networks are becoming more and more complex and highly interrelated, and it is difficult to effectively respond to the seismic capacity of today’s urban comprehensive transportation network by focusing only on the characteristics of the ground transportation network. To address the above problems, this paper proposes a method to analyze the connectivity of urban road-subway coupled transportation network under seismic effects, and quantify the connectivity performance of the network with the network efficiency as the index. The basic idea of the method is to integrate the subway stations onto the nearest road intersections, which is equivalent to adding the same number of road sections as the subway lines to form an urban road-subway coupled transportation network model. The coupled transportation network modeling is carried out through the case of Sioux Falls network and its post-earthquake connectivity assessment, and the results show that the connectivity of all three networks, namely road network, subway network and coupled network, shows a decreasing trend with the increase of peak ground acceleration (PGA). The connectivity trend of the coupled network is more similar to that of the road network. In the smaller earthquake scenario (PGA < 0.34 g), the connectivity of the subway network is the highest, followed by the coupled network, and the road network is the lowest; in the larger earthquake scenario (PGA > 0.36 g), the connectivity of the coupled network is the highest, followed by the road network, and the lowest of the subway network; and in one of the intermediate intensity earthquake scenarios (0.34 g < PGA < 0.36 g), the coupled network has the best connectivity, followed by the subway network and the lowest by the road network. Changes in the number of subway lines have an important effect on the connectivity of the coupled transportation network, and increasing the number of subway lines can significantly enhance the connectivity of the coupled transportation network.