Abstract: China’s national park scenic byways function as composite carriers that integrate ecological protection and commercial operations. Nevertheless, these scenic byways have yet to fully play their roles. Not only do some scenic byways experience frequent safety accidents, but most scenic byways across administrative boundaries also struggle to achieve unified management, and the “scenery” has yet to produce integrated benefits. Selecting Wuyishan National Park No.1 Scenic Byway and the East China K2 Line as cases for comparison, this paper conducts a comparative case study of official scenic byways and routes spontaneously formed based on tourism tracks. From the perspective of externalities, this paper constructs a theoretical framework for the unified cross-boundary management of national park scenic byways . Drawing on textual analysis, this study finds that the positive and negative externalities of national park scenic byways are unevenly distributed among different management entities due to cross-boundary differences and the accountability mechanism under the “ecological protection first” principle. This makes it difficult to achieve alignment among authority, responsibility and interests among management entities, which also impedes the formation of an effective incentive mechanism commensurate with the functions of scenic byways. The essential cause of this governance dilemma is the difficulty of controlling negative externalities and sharing positive externalities due to information asymmetry and incentive incompatibility. On this basis, this paper explores an incentive-compatible model of unified cross-boundary management. For scenic byways formed through the tourism-oriented production of traffic space, consistent management across different governments can be adopted to strengthen positive externalities. For routes spontaneously formed based on tourism tracks, it is easier to leverage market actors by introducing franchised enterprises to achieve cross-boundary management and thereby control negative externalities, making such routes a useful supplement to the national park scenic byways system.