Abstract: Within the protected ared system centered on national parks, ecosystem authenticity stands as one of the core objectives for evaluating conservation effectiveness. Many national parks are predominantly characterized by forest ecosystems. The scientific and feasible methods for assessing authenticity are crucial for examining the outcomes of national park development and supporting conservation management and spatial optimization. However, it is challenging to translate the concept of authenticity into quantifiable indicators at a regional scale due to factors such as the forest structure complexity, environmental gradient, and human disturbance. This study selects Wuyishan National Park as a case study. Authenticity in national parks refers to a state where natural succession and regeneration processes are maintained, ecological characteristics suited to local climatic and geographical conditions are preserved, the capacity to withstand natural disturbances remains intact, and natural processes dominate with minimal human interference. Based on this definition, the study develops an assessment framework for forest ecosystem authenticity across three dimensions: ecosystem natural state, species diversity, and human disturbance. The framework quantifies six indicators: forest aboveground biomass, stand origin, forest age group, dominant tree species, habitats of key protected wildlife, and human disturbance intensity. Corresponding calculation and classification methods are established for each indicator to ensure spatial comparability of data across the entire national park. The study employs an equal-weight summation method to integrate the six indicators, resulting in a comprehensive forest ecosystem authenticity index, which ensures consistency and comparability of data within the study area. The results show that the comprehensive authenticity index of the forest ecosystem in Wuyishan National Park is 0.78, indicating an overall good level of forest ecosystem authenticity at the park scale. Areas with an authenticity index above 0.8 account for 43.11% of the total park area, primarily distributed in contiguous mid- to high-elevation forest zones at altitudes of 600—1700 m. These areas feature relatively intact forest structures, strong natural regeneration capacity, and low levels of human disturbance. Authenticity exhibits distinct horizontal and vertical differentiation. Horizontally, authenticity in the eastern scenic areas and their surroundings is significantly lower than in other regions. Vertically, authenticity peaks in mid-elevation zones, while declining in low-elevation areas with intensive human activities and high-elevation sparse forest areas. Considering the practical context of conservation management and tourism development in Wuyishan National Park, the assessment results effectively reflect the current status and spatial patterns of forest ecosystem authenticity within the park. The findings provide a clear spatial basis for understanding the spatial differentiation of forest ecosystem authenticity, as well as scientific support for developing differentiated conservation management strategies in national parks dominated by forest ecosystems.