China has twin (and sometimes competing) goals for exchange rates. On the domestic front, it wants to help exporters with a cheaper currency, but it also wants to maintain a strong currency to prevent capital outflows that may weaken the country’s economy further. On the international side, China wants to avoid a trade war with the U.S., which it would have if it severely weakened the currency. It also wants to boost international use of the yuan for political purposes, as China asserts itself more strongly around the world. The country’s recent campaign to have the yuan join the mostly meaningless IMF reserve currency is one example of China desiring a strong currency. In the end, these multiple goals again promote a slightly stronger currency.
http://fortune.com/2015/08/11/why-china-devalued-yuan/