The Dayingzi Smelting Site, located at N 43°10′06.6″, E 118°26′45.1″ in Inner Mongolia, is divided into eastern and western areas. The accelerator mass spectrometry (AMS) dating of the eastern area suggests that smelting activity began from 1 000 BC at the latest, while that of the western area began about 200 years later. Furthermore, according to the lead isotope and trace element analyses and other methods, we found that the entire site used polymetallic ore from the famous Dajing Mining Site (60 km north of the Dayingzi Site) to smelt Cu-Sn-As alloy. The AMS dating results give, so far, the earliest date of the Upper Xiajiadian culture and also provide new scientific evidence for the research on the evolution of Bronze Age archaeological cultures in Liaoxi Region.