The remaining historical forests are crucial for maintaining biodiversity in urban areas. However, the integrity and stability of historical forests are affected by land-use/cover change. A better understanding of these impacts can help prioritize protection and restoration. In this study, we estimated the loss of area and levels of threats and analyzed the importance of critical areas for historical forests in the built-up area of a rapidly urbanizing city—Guiyang, China. We used the threat indicator, morphological spatial pattern analysis, and probability of connectivity based on InVEST, GuidosToolbox, and Conefor software. The results based on remote sensing image classification showed that 1988.46 ha of historical forestlands were transformed into other land-use/cover types. The mean value of the threat index of all the historical forest patches increased by 33% compared to the baseline year. The area of cores and bridges, considered as key nodes and links for the connectivity of forest networks, decreased by 193.32 and 353.61 ha, respectively. Most of the critical areas with high importance values for connectivity were located in the central part of the city and were severely threatened by the surrounding areas. We recommend that effective measures be implemented to control the further loss of historical forests and to increase the connectivity and buffering capacity of the remaining forests by creating tree belts and corridors in key locations.