The Weishan pluton is located in western Yunnan, close to the Ailao Mountain-Honghe River fault zone. It comprises monzonite porphyry and quartz monzonite porphyry. The LA-ICP-MS zircon U-Pb dating suggests that the pluton was emplaced in Eocene, with the age of (38.5±0.6) Ma. The Weishan pluton is peraluminous, with ASI from 1.09 to 1.44. All the samples have high K₂O content (4.50% to 4.98%) and K₂O/Na₂O ratios (1.10 to 1.41). The granitoids show adakitic affinities, such as high Al₂O₃, Sr, low HREE and Y content and the absence of negative Eu anomalies. The Weishan pluton has higher ε_Nd(t) and lower I_Sr values than the coeval peraluminous plutons from western Yunnan, indicating the contribution of mantle material to the formation of the pluton. All the geochemical data imply that the Weishan pluton was likely derived from partial melting of the sedimentary or metasedimentary rocks of thickened crust, triggered by underplating of basaltic magmas. Due to the early emplacement age, the Weishan pluton was not likely related to the left-lateral displacement of the Ailao Mountain-Honghe River shear zone.