To promote the coordinated development of urban rail transit with urban spatial layout, population and employment distribution, and public service facilities, this paper constructs a multi-dimensional (public transport accessibility level, PTAL) quantification model that integrates urban rail transit and conventional bus services based on a comprehensive urban transportation model. This model is used to systematically evaluate the public transport accessibility within the Dongguan metropolitan area. The study reveals that the current PTAL level in Dongguan is generally low, with average accessibility index ratios for the central urban area, sub-centers, economically strong towns, and ordinary towns recorded at 3.6:0.8:1.2:1. This indicates a spatial pattern characterized by "single-core polarization, small-scale decentralized clusters in peripheral areas, and homogeneous fragmentation in the remaining regions." The findings highlight several issues, including a disconnect between public transport resource allocation and the polycentric structure, a lack of synergy between urban rail transit and bus services, and a mismatch between public transport resource provision and the distribution of population and employment. The research demonstrates that by establishing a network of urban rail transit express lines and strengthening public transport services in underdeveloped areas, the overall public transport accessibility in the metropolitan area could increase by 65.3%. Notably, sub-centers such as Songshan Lake and Binhaiwan could experience an improvement of up to 153.6%, significantly reinforcing the polycentric structure defined as "one primary center, two sub-centers, and six clusters." Furthermore, optimizing bus connections at urban rail transit stations and coordinating the layout of public facilities can enhance the alignment between rail transit resources and the distribution of population and public services. Ultimately, this study provides technical support for establishing an integrated "rail transit+bus" system to advance polycentric development in Dongguan.