Semiconductor quantum dots (QDs) with photoluminescence (PL) emission at 900-1700 nm (denoted as the second near-infrared window, NIR-II) exhibit much-depressed photon absorption and scattering, which has stimulated extensive researches in biomedical imaging and NIR devices. However, it is very challenging to develop NIR-II QDs with a high photoluminescence quantum yield (PLQY) and excellent biocompatibility. Herein, we designed and synthesized an alloyed silver gold selenide (AgAuSe) QD with a bright emission from 820 to 1170 nm and achieved a record absolute PLQY of 65.3% at 978 nm emission among NIR-II QDs without a toxic element and a long lifetime of 4.58 mu s. It is proved that the high PLQY and long lifetime are mainly attributed to the prevented nonradiative transition of excitons, probably resulted from suppressing cation vacancies and crystal defects from the high mobility of Ag ions by alloying Au atoms. These high-PLQY QDs with nontoxic heavy metal exhibit great application potential in bioimaging, light emitting diodes (LEDs), and photovoltaic devices.