Trees possess the ability to protect outdoor cultural heritage by regulating microclimates in their surroundings. However, the specific influence of variations in tree height on heritage conservation remains uncertain. This study conducted simulations to compare the hygrothermal conditions of the Nanjing City Wall with varying tree heights (H = 7.5 m, 12.5 m, and 17.5 m) and assessed the spatial and temporal distribution of wall salt weathering risk. The results were as follows: 1) Increasing tree height led to a decrease in the average temperature and fluctuation range of the city wall surface, alongside an increase in the average surface water content. The fluctuation range of water content decreased in winter, spring, and summer but increased in autumn; 2) Higher tree height enhanced trees' effectiveness in reducing the frequency of salt weathering on the wall surface. These results provide insights for the conservation of outdoor heritage and the management of vegetation around heritage sites.