The global supply chain has faced significant disruptions, including COVID-19, Suez Canal congestion, and the Red Sea crisis, creating uncertainty in container shipping. Beyond operational metrics like throughput and efficiency, port accessibility is also crucial for stakeholders to evaluate a port's ability of dispatching and absorbing containers. To obtain port accessibility, this study proposed an analysis framework based on the spatiotemporal network derived from Ocean Alliance (OA) shipping schedules, covering 30% of global liner shipping capacity. By applying an enhanced Floyd algorithm to a large-scale three-dimensional network, the study determines the Shortest Space-time Path (SSP) between 155 ports at any departure time. Thereby, the outwards and inwards accessibilities of a certain port are quantified, defining as Export Relative Accessibility (ERA) and Import Relative Accessibility (IRA) in this work. This approach evaluates the port accessibility in the context of the spatiotemporal network, offering a better reflection on the intercoupled connections of shipping schedules. The findings offer practical insights into port performance, bridging theoretical research with industry needs and providing valuable references for shippers, forwarders, and port operators.