In the spring of 1962, a mass exodus took place in Ili Kazakh Autonomous Prefecture, the Sino-Soviet borderland in northwest Xinjiang. More than 67,000 border inhabitants, most of whom were ethnic Kazaks, managed to flee to the Soviet Republic of Kazakhstan. While the mass exodus had a wide-ranging impact, the factors that caused this incident remain debatable. Using archival documents from Xinjiang and Beijing, the author argues that both domestic and foreign causes contributed to this incident. In addition to the great famine, multiple factors played important roles in causing the mass exodus, including state policies regarding the Sino-Soviet commerce, ethnic tensions between the Han and the local ethnic peoples, political campaigns targeting at ethnic elites, and the deterioration of Sino-Soviet relations. It is also revealed that only after the 1962 incident did Beijing successfully transform the fragile suzerainty of Xinjiang's three districts, Ili, Tacheng, and Altay, into full sovereignty. In this regard, the birth of modern Xinjiang, as a frontier borderland of PRC China, occurred only in 1962 rather than in 1949.