According to the Administrative Rules of the People's Republic of China on Cross-Border Transportation of National Currency issued by China's central bank in December 2004, travelers are allowed to take up to US$5,000 equivalent of foreign currency and 20,000 yuan of local currency (US$1=7.7 yuan) into or out of the country.
There is no need to declare this to customs if the currency you carry is less than the limit.
Amounts up to US$10,000 will require a Permit for Taking Foreign Currency Out of the Customs Territory issued by a bank. Anyone who takes more foreign currency than US$10,000 is required to show a legal warrant issued by agencies of the State Administration of Foreign Exchange (SAFE)
For travelers who leave the country a second time within 15 days, the limit of foreign currency is US$1,000. If you leave the country a second time on the same day, the limit is US$500. If you have a declaration record of foreign currency when entering the country, customs will examine this before authorizing you to pass.
It is quite easy to exchange your currency into CNY, Chinese yuan, at your arrival airport, as well as hotels and banks in China, so it is not really necessary to bring Chinese currency to China.
(China.org.cn March 23, 2007)