from the St Louis Fed
For many years, China has exported more goods than it has imported. This has led to an increase in its reserves of foreign assets. In this video from the most recent Dialogue with the Fed event, St. Louis Fed Assistant Vice President and Economist Christopher Neely explains the mechanics of how China came to accumulate its significant store of foreign exchange reserves.
Source
https://www.stlouisfed.org/on-the-economy/2016/may/how-did-china-build-up-reserves
Disclaimer
Views expressed are not necessarily those of the Federal Reserve Bank of St. Louis or of the Federal Reserve System.