Abstract: The Yelasai copper deposit, Qinghai Province, is located in the west of eastern Kunlun orogenic belt. It is a newly discovered, medium-sized copper deposit in the Qimantage area. The major host rocks are the intermediate acid intrusive rocks of Early Indosinian, the minors are marble and basic volcanic rocks of Tanjianshan Group. Mineralization is controlled by NWW-striking faults, showing hydrothermal alteration of silicification, sericitization and potash feldspathization. According to the crosscutting relationships of the vein bodies and mineral replacement, the ore-forming process can be divided into three stages: StageⅠ, Ⅱ and Ⅲ. Research on fluid inclusions shows that: fluid inclusion types of ore-bearing quartzes can be further classified into three subtypes: Aqueous and gas two-phase (W-type), daughter mineral-bearing three-phase (S-type) and CO2-rich three-phase (C-type). The salinity of hydrothermal fluid is 3.42%~65.48% NaCleqv, mainly concentrated in 40.00%~55.00% NaCleqv and 4.00%~10.00% NaCleqv. The density is 0.43~1.25g·cm-3, concentrated in 1.07~1.14g·cm-3 and 0.55~0.85g·cm-3, both of them show two peaks on the histogram. The whole range of mineralization temperature is 187~579℃, whereas temperature in stageⅠ, Ⅱ and Ⅲ is 332~579℃, 294~345℃, 187~212℃, respectively. Coexisting of C-type and S-type inclusions in stageⅠindicates fluid-boiling or immiscibility. Possible fluid evolution is as follows: The original magmatic ore-forming fluids, extremely high temperature, medium to high salinity (about 15%~25% NaCleqv), directly exsolve from a melt in the final stage of magma crystallization and partition into immiscible S-type(high salinity)and low salinity C-type and low salinity W-type fluids, which are trapped at high temperature. Fluid-boiling or immiscibility during stageⅠcauses ore precipitation for the first time, while fluid-mixing during stageⅡplays an important role in Cu deposition. Mineralization pressure is about 47.4~93.1MPa, estimated by isochore graphic method, and the depth is 1.8~3.5km. The acid intrusive rocks, closely associated with mineralization, belong to high potassium, peraluminous calc alkaline series. The contents of REE range in 133.9×10-6~240.6×10-6, between the average of upper curst and that of the lower crust, LREE fractionation is significant while HREE fractionation is not, negative δEu anomalies are evident. The features in HREE loss, K, Rb, Th positive anomalies and Nb, Ta negative anomalies, particularly P, Ti loss, share similar features with the granites in collision belt in the terms of geochemistry. In the Rb-Y+Nb and Nb-Y diagrams, all samples fall in syn-collision granite field. Genetic type of Yelasai copper deposit is porphyry-like copper deposit under the control of linear faults, mineralization occurs during the intensive intracontinent orogenic period in the late stage of Early Indosinian.