This paper is the result of geological survey engineering.[Objective] The formation mechanism of the Xiaowan granitic pluton, located in the magmatic belt of the Lincang terrane in the Fengqing County, western Yunnan province, is very important for constraining the petrogenesis of the Lincang magmatic arc derived from crust, subduction of the ocean basin, and the time of collision closure. [Methods] This paper reports for the first time the zircon U-Pb chronology, petrography, and geochemical characteristics of major and trace elements in the Xiaowan granitic pluton. [Results] The Xiaowan granite is mainly composed of biotite monzogranite. Zircon U-Pb LA-ICP-MS dating shows that its formation age is (260.4±1.3) Ma (MSWD=0.32, n=18), suggesting that the Xiaowan granite was formed in the Late Mid-Permian, rather than the Middle Proterozoic as previously considered. Whole-rock geochemical analysis shows that the A/CNK of biotite monzogranite is greater than 1 (1.02-1.47), indicating that the Xiaowan granitic pluton is composed of weakly peraluminous granite. These rocks are characterized by high silica (SiO₂ = 67.54%-78.66%), alkali rich (K₂O+Na₂O = 6.20%-8.52%), poor in the elements of calcium (CaO=0.25%-3.20%), magnesium (MgO content 0.11%-1.95%) and phosphorus (P₂O₅ content 0.03%-0.16%). Meanwhile, they are enriched in large ion lithophile elements (LILE) such as Rb, Th, Ce, relatively depleted in high field strength elements (HFSE) such as Ta, Nb, Zr, Ti, Hf, Y. In the (Y+Nb) vs. Rb, Yb vs. Ta diagrams, they exhibit dual geochemical characteristics of arc and collisional granite. [Conclusions] Based on regional geological data, we suggest that the Xiaowan granitic pluton is the product of partial melting of the mudstone mixing with sandstone at the upper crustal level, under the setting of the eastward subduction shifting to the convergence of the Changning-Menglian ocean (also called as “Three Rivers” ancient Tethys ocean). This study provides important chronological and petrological evidences for the transition of the Changning-Menglian ocean from arc-continental subduction to collision in the southwestern Qinghai-Tibet Plateau.