Abstract: A set of basic dyke intrusion has been found in metamorphic rocks of Tuolai Paleoproterozoic rock group in Yulongtan Region. Gabbro vein and diabasic dyke are found to be the main intrusive rocks and they intruded into the gneiss and schist of Tuolai rock group. The axes of the dykes are mostly NW, with a few close to SN or NE, the outcrop scale is within 10-35m in length and 5-10m in width. Isotope dating analysis on diabasic dyke shows its geological age as early Permian. Combined with its geological features, petrology, petrochemistry and geochemistry characteristics, it is suggested that the dyke belongs to tholeiitic series and forms in extensional tectonic setting. Considering the regional geological analysis, it is concluded that after Caledonian orogeny, Qilianshan orogenic belt had experienced a stretch and collapse due to the thickening of continental crust, and the process led to the underplating of basic mantle-derived magma, thus formed the basic dyke. It is reflected that in early Permian a basic magma underplating event occurred with continental crust thickening and extension in north Qilian area.