Neoproterozoic magmatic rocks are developed along the Jiangshan—Shaoxing fault zone. They give us a window to understand the Neoproterozoic tectonic evolution, and the aggregation and breakup of the Rodinia supercontinent. This paper reports the results of SIMS zircon UPb age and wholerock geochemistry of the Lilong syenite in western Zhejiang. The Lilong syenite has high SiO2 content (average of 59.65%) with low FeOT and MgO content (average of 4.06% and 1.41%). The ratio of Na2O/K2O ranges from 3.27 to 5.34, with an average of 4.46. The aluminum saturation index (A/CNK) is between 0.85 and 0.94, resemble the characteristics of calcalkaline quasialuminous Itype granite. The syenite is enriched in large ion lithophile elements such as K and Ba, high field strength elements (HFSE) such as Th and Ce and light race earth elements (LREE), and relatively depleted in HFSE such as Ta, Nb, Zr and Ti and heavy race earth elements (HREE), which is basically consistent with the Andeantype calcalkaline series rocks. The rock samples have a slight negative Eu anomaly with the average value of 0.82 (δEu 0.77~0.87), indicating a small amount of crystallization differentiation of plagioclase. The SIMS zircon UPb dating result of syenite is 840.7±2.4Ma, which belongs to the product of magmatic activity in the middle Neoproterozoic. Based on the results of this study, the Lilong syenite may be formed in the continental backarc environment generated by the northward subduction of the ancient oceanic crust to the Yangtze Block. The magma source of the Lilong syenite origin from the lower crust. Combined with the regional tectonic evolution background, it is proposed that the Yangtze and Cathaysia Block have not finally collided at ~840Ma, and the adjacent area, such as the Zhejiang, Anhui and Jiangxi, still in the subduction zone background during the continuous subduction of the ancient South China Ocean Plate to the southeastern margin of the Yangtze Block in the middle Neoproterozoic. This process may be related to the Rodinia supercontinent convergence event.