The accelerated development of the Guangdong-Hong Kong-Macao Greater Bay Area has created a new spatial carrier and institutional foundation for Hong Kong and Macao seniors to receive cross-border elderly care in the Chinese mainland. Driven by structural pressures such as rapid aging, high housing prices, and strained long-term care resources in Hong Kong and Macao, and supported by institutional innovations such as the Guangdong Residential Care Service Scheme and the Guangdong Scheme, the scale of cross-border elderly care for Hong Kong and Macao seniors has expanded steadily. However, practical development is still faced with multiple challenges, including inadequate welfare connectivity, bottlenecks in cross-border medical security, large differences in service standards, and insufficient cultural adaptation and social support. Based on the theories of population migration and elderly welfare economics, this paper systematically analyzes the four-dimensional influencing factors of cross-border elderly care decisions for Hong Kong and Macao seniors, namely economic and cost factors, living environment and service supply, institutional and welfare portability, and family and social support networks. It also sorts out the innovative exploration of Guangdong, Hong Kong, and Macao in welfare portability, cross-regional collaborative governance, medical-care integration, and smart elderly care, and puts forward targeted policy suggestions for optimizing the cross-border elderly care ecosystem. The research aims to provide a theoretical reference for improving the long-term stability and equity of cross-border elderly care in the Greater Bay Area and promoting the high-quality development of the regional elderly care service system.