A currency devaluation of yuan helps countries sell more exports, boosting the economy and help its own manufacturers .Right now the Chinese economy is in the midst of an economic slowdown and has suffered from stock market turmoil, so it can use some extra help.
and Chinese devaluation have affect to other countries such as US exports.
Of course, everything I've just said works in reverse for the United States. As the yuan gets cheaper from the perspective of American consumers, the dollar gets more expensive from the perspective of Chinese consumers. That means it's getting more expensive for Chinese people to import American-made goods, so they're likely to import fewer of them. Lower demand for US goods could mean slightly slower economic growth here in the US. (And the same, of course, is true of other countries whose currencies are gaining value relative to the yuan.)
By devaluing its currency, China intends to boost its exports to regain competitiveness, but economists say the move will hurt U.S. exports to China.
The devaluation will make Chinese-made goods less expensive, and imports to China more expensive. Economists say this should support growth in China, which has slowed in 2015, in large part because of a cooling real estate market.
For the U.S., however, trade already is expected to be a hindrance on growth in the second half of this year, and that drag will now be somewhat larger, Gus Faucher, senior macroeconomist at PNC Financial Services Group, said in a note.