To put things in perspective, China holds approximately 7.5% of the total US debt load ($17.3 trillion). That's an estimated $1.3 trillion in U.S. Treasuries. They are the largest foreign investor in US treasuries's[0] and the major holders of all USD foreign exchange reserves due to their huge trade surplus with the US. These dollars need to be invested somewhere and the U.S. Treasury market is one of the few places that China can recycle its surplus dollars.

Even during the financial crisis in 2009 and the debate around the public debt ceiling in the summer of 2011 with the subsequent downgrade of the U.S. long-term sovereign credit by S&P, China continued to increase their USD reserves while other endowment funds in regions such as the Middle East took steps to diversify their FX reserves away from the USD. China is dependent on the US Treasury market - and it's stability - because as long as China continues to hold down the value of its currency to the USD, it will have few options other than to keep investing in U.S. dollar assets.

[0] http://www.treasury.gov/resource-center/data-chart-center/ti...