The article is devoted to the history of the Oirat migration process from Central Asia to the South-Western Siberia. Due to the lack of sources this subject has not been sufficiently studied on many aspects in the historiography. The author identifies four main reasons for migration: environmental, internal political situation, foreign policy, and economic. The main assumption is that revealing the reasons of the Oirat migration from Central Asia to Siberia will allow to resolve all the issues of political history of the Oirats in the 17th century. The mass migration of the nomads from Central Asia to the west has long been seen in history. Nomads being pastoralists, hunters, and warriors due to the dominant mode of production were forced to search for vast areas. Population growth, increase in the livestock, unsuccessful wars, decrease of pasture areas and others forced them to embark on long wanderings. One of which was made by the Oirats in the 17th century. Kalmyk uluses, which had migrated from Western Mongolia, located in the upper reaches of the rivers Irtysh, Ishim, near Kamyshlov town, Omi and Salt lakes. Kalmyks became nearest neighbors of Siberian uyezds (districts) of Muscovy.