Using zircon SHRIMP U-Pb dating method in this paper, we conduct the SHRIMP UPb ages of tuff at the bottom of the Nanying’er Group, Wangjiashan area, Jingyuan County, Gansu Province and acquire a set of weighted average age of (244.9±2.8)Ma. Combined with the feature of Cathodoluminescence images and Th/U specific value of zircon, we believe that the sedimentary age of Wangjiashan tuff is (244.9±2.8)Ma. High precision and high confidence isotopic age data fully demonstrate that the bottom of Nanying’er Group exists with the early of middle Triassic strata. The author proposes that the Wangjiashan tuff and its hereinafter strata should be incorporated in Dingjiayao Formation. Meanwhile, it can be confirmed that the Wangjiashan Tuff and the Baojishan tuff are the deposition product of the same volcanic eruption. It can be used as a marker bed of the regional stratigraphic division and correlation. The above research results are beneficial for the upper and middle Triassic stratigraphic division and correlation of Hexi corridor region, and its stratigraphic significance for the final establishment of Triassic chronostratigraphic division and correlation framework of China’s northern continental strata.