At a national scale, exploring the transportation accessibility of megalopoleis effectively depicts their collaborative relationships and trends in intra-cluster integration, providing theoretical support for promoting their integrated development. Based on an enhanced time-space map analysis model, this study measures the accessibility levels of 11 national-level megalopoleis and analyzes the evolutionary characteristics of accessibility within and between clusters under the construction of a high-speed rail networks. Results demonstrate that the ‘space-time compression’ resulting from the construction of a high-speed rail networks facilitates the diffusion of megalopoleis from a ‘two horizontal and two vertical’ distribution structure to a ‘clustered’ structure. The construction of a high-speed rail networks promotes intra-cluster integration and inter-cluster collaboration, thereby enhancing the regional balance. The time-space map analysis model effectively interprets the spatial information of the time-space map, expands the application of spatial analysis methods for accessibility, and provides insights for promoting balanced regional development in megalopoleis.