The light yellowish-green volcanic tuff beds, usually called "Green bean rock", distribute widely in southwestern China, but whether these beds are synchronous across the Upper—Lower Yangtze Platform and Nanpanjiang Basin remain unclear. In this study, we focus on the radiogenic isotope age of the volcanic tuff in Shimenkan Section and intend to compare the isotopic ages between the Upper—Lower Yangtze Platform and Nanpanjiang Basin. Methods: Four volcanic tuff samples (SMK01-04) were collected from Layer 0 to Layer 3 of the bottom of the Guanling Formation in Shimenkan Section. The samples were concentrated and separated by standard techniques of density and magnetic separation at Dishi Technique Company in Beijing. The LA-ICP-MS analysis was carried out in Mineral Laser Microprobe Analysis Laboratory (Milma), China University of Geosciences (Beijing). The internal structure of the zircons was examined using cathodoluminescence (CL) before isotopic analyses. 32 zircons with clear oscillatory zoning were chosen for the analyses in each sample. Uncertainties for each analysis were reported at the 1σ level. Weighted mean ages of pooled analyses are quoted at the 95 % confidence interval. Data reduction and U-Pb concordia diagram were carried by Isoplot. Results: Four concordant ages were obtained from each sample, as 246. 9±1. 1 Ma (MSWD = 0. 58) for SMK01, 245. 0±1. 3 Ma (MSWD = 0. 42) for SMK02, 245. 8±1. 7 Ma (MSWD = 1. 11) for SMK03, and 243. 9±1. 6 Ma (MSWD = 0. 37) for SMK04, respectively. Conclusions: The age of SMK01 is similar to that from the volcanic tuff layer at the bottom of the Guanling Formation at Zhangkoudong Section, Yunnan, which means this layer could be well correlated within the Upper Yangtze Platform. This age is also comparable to the age of the tuff at the bottom of Badong Formation on Lower Yangtze Platform and the tuff at the Transition Beds above Luolou Formation in Nanpanjiang Basin, which made it a potential mark of the base of the Anisian in southwestern China. The ages of SMK02-04 represent multiple volcanic events within the Middle Triassic. These tuff beds may not only constraint the Olenekian—Anisian Boundary, but also provide potential nutrition which fertilized the primary productivity in the ocean and accelerated the recovery of marine ecosystem in Middle Triassic thereafter.