There exist two different viewpoints concerning the bottom age of Nanhua System. i.e., early age (780 Ma or 760 Ma) and late age (725 Ma or 720 Ma). Zircon LA-ICP-MS U-Pb dating of a tuff sample from the upper part of Yanmenzhai Formation in Dongqiao, Pingxiang, was carried out, and 28 analyses of zircons show five main age groups. Then the authors probed into the geological settings of the five age groups and reached the following conclusions. The first and the second age groups with weighted mean age of 805.5±10.6 Ma and 767.6±7.7 Ma respectively came from inherited magmatic zircons, and were related to the island-arc magmatism on the southeastern margin of Yangtz Block during 810～800 Ma and the magmatism of 760 Ma in middle Banxi Period respectively. The fourth and the fifth age groups with weighted mean age of 390.8±2.8 Ma and 211.2±1.5 Ma respectively came from metamorphic zircons, and were related to the late Caledonia tectonic-hydrothermalism and the late Indonian tectonic-hydrothermalism respectively. The third age group with weighted mean age of 717.2±8.9 Ma came from 11 magmatic zircons and represents the formation age of the tuff. The age of 717.2±8.9 Ma provides a constraint for the bottom age of 720 Ma of Nanhua System. In addition, the authors studied the zircon U-Pb ages of around 760 Ma which came from the lower part of Chang'an Formaion in Bixi, center Hunan; it is pointed out that the ages came from inherited zircons, and that the upper limit age of 720 Ma held by 50 percent of the zircons suggests that the rock-forming age is 720 Ma rather then 760 Ma.