Abstract: Tangwangzhai-Yangtianwo compound syncline is a typical slip nappe structure in the Longmen Mountain foreland structural belt in Western Sichuan.Six diabase dikes are developed in its northeast part, and limiting its intrusion time is helpful to understand the tectonic evolution of the northern Longmen Mountain.In order to determine the invasion time of diabase dyke, this paper defines the geological age of the lower limit of diabase dyke invasion through field geological investigation and the relationship between the formation, evolution, superposition and cutting of regional structures.The results show that: ①In the complex fault fold belt in the front of Longmen Mountain, the Late Triassic Xujiahe Formation and Jurassic continental clastic rocks are in parallel or angular unconformity contact, which are superimposed on the Devonian, Carboniferous, Permian and early Middle Triassic marine strata, The Tangwangzhai-Yangtianwo compound syncline was formed in the Indosinian-Yanshanian period after the Middle Triassic; ②Tangwangzhai-Yangtianwo compound syncline is located on a large area of Silurian weak shale, and no fault scratch is found on the contact surface.The plastic rheology of the underlying Silurian is the main factor for the formation of the syncline prototype; ③Because diabase dykes cut the Tangwangzhai-Yangtianwo compound syncline, their intrusion age should be after the deposition of the Leikoupo Formation in the Middle Triassic.225~158 Ma should be the distribution range of intrusion age of diabase dyke.