Abstract: The Fengning ancient pine (Pinus tabuliformis Carrière) which commonly known as Fengning Jiulong pine was planted in the middle of the Northern Song Dynasty and has a history of nearly 1000 years. In recent years, the sprouting ability of branches has weakened, and the amount of shoots has decreased, indicating weakness of the tree. To analyze the reasons for the weakness of Fengning Jiulong pines, instruments such as stress wave wood detector and root ground penetrating radar were used to detect the main trunk empty decaying, branch bark damage, and main root distribution areas. The distribution and growth status of the root system were detected using soil profiling methods, and the main physical and chemical properties of the soil in the root distribution area were tested in the laboratory. The results showed that the main trunks of Fengning Jiulong pine were inclined, with a heavy degree of hollow rot, and the maximum hollow rot rate in the section reached 44%. Some repaired spots on trunk and main branches cracked again; Roots with diameter of 2 cm or more are mainly distributed within the vertical projection range of the tree crown. The soil bulk density in the habitat is relatively high, and the content of major nutrients is low. The alkaline soil, high bulk density, and lack of nutrients in the habitat are the main reasons for the decline of tree vigor.