Garciyunnanol A (1), an unprecedented 1,2-seco-bicyclic polyprenylated acylphloroglucinol (BPAP) possessing a unique 6/6/6 tricyclic core, was characterized from Garcinia yunnanensis together with 16 BPAPs, including eight new compounds (garciyunnanols B–I, 2–9). Biogenetically, the bicyclo[3.3.1]nonane-2,4,9-trione moiety of 12 reconstructed the bicyclic δ-lactone core of 2 through Norrish type Ⅰ cleavage and cyclization, followed by a cyclization of two side chains to form an intriguing 6/6/6 tricyclic core of 1. Their structures were elucidated through analysis of spectroscopic data, calculation and comparison of ECD spectra. Bioactivity evaluation manifested that compounds 1, 2, 5, 6 and 14 demonstrated superior inhibition of NO production compared to the positive control dexamethasone. Notably, compound 5 exhibited a dose-dependent inhibitory effect on NO production, with an IC₅₀ value of 0.25 ± 0.87 µM. Furthermore, experiments involving ELISA, Western blotting, and immunofluorescence staining revealed that 5 effectively reduced the secretion of interleukin-1β in LPS plus nigericin-stimulated THP-1 macrophages by inhibiting the activation of the NLRP3 inflammasome.