The Xuji deposit is an important Ag-Pb deposit found in the Zongwulong belt. Geologically, the ore deposit is hosted in the Carboniferous-Permian Guokeshan Formation of Zongwulong Group, and consists of limestone intercalated with phyllite. The orebodies are veinlike, lenticular and stratoid in shape and are controlled by EW-trending faults. The alteration of ore-bearing wall rock is mainly manifested as silicification and sericitization. Ore minerals are mainly galena and silver tetrahedrite, gangue minerals are mainly quartz, and the ore exhibits variegated and reticulate structure. In this paper, the authors analyzed S and Pb isotopic composition characteristics in this deposit. The results of sulfur isotope analysis show that the δ³⁴S values of sulfide minerals in the ores range from 5.0‰ to 8.4‰, indicating that the sulfur mainly came from magma with a small stratigraphic contribution. The lead isotopic compositions, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb are from 17.896 to 17.922, 15.589 to 15.617 and 38.072 to 38.166 for ore sulfide minerals, respectively, which are similar to the features of wall rocks (²⁰⁶Pb/²⁰⁴Pb=17.949~18.976, ²⁰⁷Pb/²⁰⁴Pb=15.600~15.696,²⁰⁸Pb/²⁰⁴Pb=38.106~40.943) and are different from the features of Xujixiakou diorite (²⁰⁶Pb/²⁰⁴Pb=18.144~18.589, ²⁰⁷Pb/²⁰⁴Pb=15.623~15.636, ²⁰⁸Pb/²⁰⁴Pb=38.790~39.033), indicating that the metals were mainly from the Guokeshan Formation of Zongwulong Group. A comprehensive analysis shows that the Xuji deposit was related to the late Permian magmatic activity in the Zongwulong belt and the genesis of the deposit seems to be magmatic hydrothermal filling-replacement.