Abstract: The magmatic event of Yanshanian period in South China has been divided into two stages:Early Yanshanian (180-150Ma) (J2-J3) and Late Yanshanian (140-80Ma) (K1-K2). However, there exist only the data concerning Cretaceous mineralization associated with acidic intrusive rocks, but the record of the early magmatic events on the southwest margin of the Yangtze Plate has not been seen. In this paper, the authors newly discovered Maidicun lamprophyre in Jianshui area of eastern Yunnan. The U-Pb age of the zircon is 158.4±2.4 Ma (MSWD=4.2), and hence it is obvious that the lamprophyre was formed in the Late Jurassic, corresponding to the first stage of the thermal event in southeast China. The values of incompatible elements are significantly higher than those of the primitive mantle of Maidicun lamprophyre, showing different degrees of enrichment. The LILE (Rb, Ba, Ce) enrichment is not obvious, HFSE (U, Ta, Pb) show some enrichment, and Nb, Ti, Zr, Hf don't show significant enrichment and losses. LREE/HREE ratios are in the range of 5.65-6.07, averagely 5.89,(La/Yb)N ratios are in the range of 7-12, indicating LREE enrichment and HREF relative losses. It has no negative anomalies of Nb-Ta-Ti and shows REE characteristics of OIB-type oceanic island basalts, and the Maidicun lamprophyre was formed under the condition of intraplate extension. NNE-trending structures were developed in the study area during the Mesozoic, and the early NNE-trending folds were superposed by late normal faults in the same direction. The fold axial surface is mainly SE-trending and partly NW-trending, indicating tectonic characteristics in response to SEE-NWW trending extrusion stress. Faults are mostly high-angle normal faults in the NW-SE direction, showing that they were influenced by EW-trending tensile stress. The lamprophyre's intrusion was mainly controlled by the action of deep asthenosphere mantle, from which the fluid experienced metasomatism with lithosphere mantle in response to partial diapering of asthenosphere and the stretch of lithosphere in eastern Yunnan in the Late Jurassic period, followed by the formation of the enriched mantle. And this kind of enriched mantle was partially heated, melted and migrated upward for emplacement finally. The intrusion age of the lamprophyre (Late Jurassic 158 Ma) is the starting time of extension in the study area.