The eastern margin of the Qaidam Basin lies in a key tectonic location connecting the Qinling, Qilian and Kunlun orogens. The Permian – Triassic (P-T) I-type granitoids in the basement of the eastern Qaidam Basin are parts of the intrusive magmatism of the Paleo-Tethyan orogeny formed in ocean environment. Lithogeochemical analyses, combined with previous LA ICP-MS zircon U-Pb dating, reveal two time series of P-T granitoids. The early series is composed of K-feldspar granites, granodiorites, tonalities, and diorites, and the later series is composed of alkali granites, K-feldspar granites, and granodiorites. Both of them have lithogeochemical trends of anti-Bowen’s reaction series, opposite to that of classical Bowen’s reaction series. Nd-Sr-Pb-O isotope compositions of P-T granitoids showes characteristics of young crust formed in orogeny and upper crust with a close relationship to enriched mantle (EM II) source. The Pb composition of P-T granitoids from the basement of the eastern Qaidam Basin is very similar to that of the Mesozoic granitoids from West Qinling Orogeny and Yangtze Block, suggesting the tectonic affinity between the basement of the eastern Qaidam Basin and the Yangtze Block. It is suggested that, underplating of mafic magma from the mantle and dynamic mixing between the mantle-derived mafic and crust-derived felsic magmas, may act an important role in the crust growth of the basement of the Qaidam Basin.