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 effects 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.)
IT'S AS IF EVERY BUSINESS IN CHINA CUT ITS PRICES FOR AMERICANS BY 4 PERCENT
For this reason, people often treat currency devaluation as a "win" for the devaluing country and a "loss" for the country whose currency gets more valuable. That's why Chinese officials have had to defend themselves against criticism from abroad — people outside China worry that further declines in the yuan could weaken economic growth outside of China.
But it's important not to forget that the first-order result of a cheaper yuan is that American consumers pay lower prices for Chinese goods. That's good for American consumers. So if the Fed can offset the negative macroeconomic effects (which it probably can right now — more on that below), a cheap yuan could be good for Americans overall.