Abstract: China’s national park scenic byways, as composite carriers that combine ecological protection and operation functions, have not performed satisfactorily. Some scenic byways experience frequent safety accidents, and most across-administrative-boundary scenic byways find it difficult to achieve unified management. This paper selects Wuyishan National Park No.1 Scenic Byway (an officially planned and constructed national park scenic byway) and the East China K2 Line (a spontaneously formed national park scenic byway) as typical comparative cases. Based on externality theory, it constructs an analytical framework for the cross-boundary unified management of national park scenic byways, and conducts analysis by combining policy documents on the Wuyishan National Park belt, travelogue texts from the Two-Step Road website, and on-site interview materials. The study finds that, under the principle of "ecological protection first," differences in accountability mechanisms across different administrative districts lead to an uneven distribution of the positive and negative externalities of national park scenic byways among different managers, making it difficult to achieve a correspondence between managers' rights, responsibilities, and interests, and difficult to form effective incentives commensurate with the full realization of scenic byway functions. The institutional causes of national park scenic byway management problems can be summarized as information asymmetry and incentive incompatibility, which make negative externalities difficult to control and positive externalities difficult to share. Accordingly, a cross-boundary unified management model that can achieve incentive compatibility is proposed: for officially planned and constructed national park scenic byways, positive externalities can be strengthened through standardized and consistent management among government authorities in different regions; for spontaneously formed national park scenic byways, cross-boundary unified operation can be achieved by relying on national park commercial service providers to control negative externalities, making them a beneficial supplement to the national park scenic byway system.