Abstract: Against the rapid development of the low-altitude economy and the shift toward whole-territory, full- element governance of territorial space, low-altitude airspace is increasingly recognized as a new type of three-dimensional territorial spatial resource. Traditional low-altitude airspace delineation, however, has long prioritized airspace operations while underemphasizing ground-space functions, making it difficult to respond to diversified low-altitude activities such as logistics delivery, emergency rescue and passenger flight. From an air-ground system-coupling perspective, this paper argues that low-altitude airspace delineation should shift from route control to three-dimensional spatial governance. It clarifies the territorial-resource attributes of low-altitude airspace, reveals the coupling between aerial access and ground use, constructs a technical system for coordinated air and ground boundary delineation, and proposes a usage-management framework combining use zoning and use classification. A case study of Jinan verifies the scientific validity and operability of the framework. The study provides methodological support and decision-making reference for the efficient allocation of low-altitude resources and the high-quality development of the low-altitude economy.