Abstract: The formation age and geochemistry of the diorite, was firstly found in Suli area of western section of Central Qilian by 1:50000 regional geological survey, have been studied and analyzed in this paper. The zircon U-Pb dating shows that this diorite was formed in Late Jurassic, with an age of (155.1+1.4) Ma. The geochemical data suggests that Al₂O₃, Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O and K₂O contents are 13.37%~13.55%, 5.37%~5.90%, 5.64%~6.20%, 0.11%~0.16%, 1.85%~2.16%, 5.15%~5.67%, 3.13%~3.58% and 1.39%~1.54%, respectively, with a geochemical characteristics of rich Fe and Ca, but poor Si and Al. Its σ value and A/CNK ratio range from 1.32 to 1.41 and from 0.75 to 0.83, respectively, showing the feature of quasi-aluminum cal-alkaline series. Its ΣREE value, LREE/HREE ratio and δEu content vary from 291.44×10^-6 to 319.87×10^-6, from 4.93 to 5.06 and from 0.79 to 0.86, respectively, having obvious characteristics of crust source magmatic rocks. Its trace elements have obvious negative anomalies of Ba, K, Sr, and Ti, while Rb, Th, Ce and Nd are relatively enriched. Combined with the lithogeochemical characteristics and the discrimination diagrams of tectonic setting, the results can determine that this diorite was formed in the relaxation phase after the collision, serving as the product of plate tensional environment. The determination of zircon U-Pb age and genesis of this diorite provide an important geological information for the further study of the tectonic evolution of the Qilian orogenic belt.