The Early Cretaceous Jiashan syenite is located in a typical crustal thickening region, where the Late Jurassic Chengde thrust sheet was reported in the eastern segment of the Yanshan intracontinental erogenic belt. In accordance with their intruding order in time from earlier to later, the syenite can be divided into three units, the Jiangjiawan, Longtangou and Longtannangou units. Geochemically the Jiashan syenite is rich in Si, alkali, Fe, REE, Th, Ca, Nb, Zr and Hf, poor in Mg, Ba, Sr and Ti, and depleted in transitional elements such as Cr, Co, Ni, and V, with a high Ga/Al ratio. Its Ce/Pb ratio ranges from 6. 12 to 13. 41 and it is enriched in light REE (LREE) with a moderate Eu depletion. The Jiashan syenite, a post-orogenic intrusive, can be considered as an A-type granite, emplaced during a mantle uplifting in lithospheric extension, which suggests that the regional compressional shortening and crustal thickening ended and the lithospheric extension was prevailing. Therefore, the petrology and geochemistry of the Jiashan syenite document a lithospheric thinning in North China, which started in the Early Cretaceous.