This paper reports four in-situ zircon LA-ICP-MS U-Pb dating results to precisely constrain the sedimentation time of the Fanjingshan Group. CL images reveal that the fine-grained zircons from these samples have similar internal structures. LA-ICP-MS U-Pb analyses indicate that all the zircons without zoning have ²⁰⁶Pb/²³⁸U ages older than 1.0 Ga. On the other hand, the zircons whose zoning has been destroyed yielded unusually low ²⁰⁶Pb/²³⁸U age value down to 50 Ma. Analyses of other grains yielded accordant ²⁰⁶Pb/²³⁸U ages, showing slightly different weighted mean ages of various samples. The new data, along with previously published data, demonstrate that the Fanjingshan Group was deposited during the period of 850～815 Ma, which precisely constrains the timing of the Wuling/Sibao Movement, provides critical data for geological correlation, and helps understand the possible tectonic evolution of the Jiangnan orogenic belt.