China's interior consumption only represents 30% of its GDP, it's economical growth is not based on its interior consumption but on its exportation, a big part of its consumers are the USA. China has a huge commercial surplus due to the fact that it exports way more than it imports and its main consumers are the USA, with this surplus China chooses to buy US debt to maintain their current consumption and thus maintaining its current economical situation.

Sorry if this is not really comprehensible I am studying in french and it's hard to get the right terms in english.

Edit : 
Here are some stats on which this theory is based http://www.worldstopexports.com/chinas-top-import-partners/