The Paleo-Tethyan granitoids in eastern Tibet are generally Cu-barren, and only a few can generate porphyry Cu deposits. It is still unclear what magmatic differences cause these coeval granitoids to have different mineralization potentials. In this study, the major, minor, and trace element abundances and Nd isotope in the apatites from four Triassic granitoids from eastern Tibet have been determined using EPMA, LA-ICP-MS, and LA-MC-ICPMS. The following are the selected plutons: the Pulang pluton, which is associated with the porphyry-type Cu deposit, and the coeval Cuojiaoma, Dongcuo, and Daocheng plutons, three of which do not generate any Cu mineralization. Our study reveals that Ga, Ce, S, Cl, and Cu in apatite are effective indicators for both magmatic oxidation states and the abundances of volatiles and ore-forming metals. The Nd isotope of apatite can be used to trace the magma source. According to the above chemical compositions of apatite and Ce4+/Ce3+ ratio of zircon, the parental magma of the Pulang pluton is identified to be more fertile for Cu mineralization than those of the other three plutons, i.e., Cuojiaoma, Dongcuo, and Daocheng, because it is more oxidized and richer in Cu, Cl, and S. The aforementioned differences in magmatic physical and chemical conditions among the plutons investigated are ascribed to the different sources and natures of magmas, as revealed by both the Nd isotope and delta Eu, Sr/Y, and La/Yb values of apatite. Generally, the results indicate that magmatic fertility mainly governs the Cu-mineralization potential of Paleo-Tethyan granitoids in eastern Tibet, and that apatite is a robust probe for magmatic fertility.