Ecological security is a crucial component of the national security system, directly influencing resource security, food security, and climate security. As a key concept linking ecosystem stability with human sustainable development, its connotation has evolved from static structural representation to dynamic spatial processes characterized by multi-scale coupling. This study systematically reviews the conceptual connotation and developmental trajectory of ecological security in China. From the perspectives of externalities and ecosystem services, the study analyzes the spatial spillover effects and transmission mechanisms of ecological security, emphasizing that ecological security issues are essentially the cross-scale mismatch and transfer of ecological impacts. Based on these multi-scale characteristics, this paper adopts the watershed as the spatial analysis unit to investigate the spatial associations and spillover effects of key ecosystem services, including water retention and soil conservation, under watershed-scale analysis. Using the Poyang Lake Basin as a case study, the paper reveals the multi-level coupling characteristics of ecological security needs, demonstrating how lakes, watersheds, and river-lake systems interact to maintain regional ecological security while supporting biodiversity conservation. The study further emphasizes that effective ecological security governance must transcend single-scale thinking to develop a cross-scale collaborative governance framework. By coordinating ecological function zones, watershed units, and administrative boundaries, and utilizing tools such as ecological compensation and policy integration, it is possible to enhance the systemic and adaptive capacity of ecological security governance. Additionally, it argues that watershed-based governance is becoming an effective approach for ecological security management due to its natural boundaries, systematic structure, and the capacity to reflect interactions between ecological processes and human activities. This research provides theoretical support and practical guidance for advancing systematic ecological security governance and policy management in China and offers insights for global regions facing similar cross-scale ecological security challenges.