Chinese customs runs the familiar two channel system, and the thresholds are specific enough to be worth reading before you pack rather than after.
The channels
Use the green “Nothing to Declare” channel if you have nothing to declare, and the red “Goods to Declare” channel if you do.
If you are not sure, take the red channel or ask a Customs officer. That is the official instruction, not a cautious suggestion, and it is far cheaper than guessing wrong in the green lane.
Passengers under 16 traveling with adults are exempt from completing the baggage declaration form.
The general test
All baggage and personal articles must be for personal use and within a reasonable quantity. “Reasonable” is doing real work in that sentence: quantity is what separates personal effects from an import.
Declare above these amounts
Alcohol and tobacco. Declare 1,500ml or more of alcoholic drinks at 12% alcohol or above, 400 or more cigarettes, 100 or more cigars, or 500g or more of tobacco.
Cash. Declare RMB 20,000 in cash or above, or foreign currency in cash equivalent to US$5,000 or above.
The one that is a hard limit rather than a declaration
The limit of RMB you may bring into or take out of China is 20,000 yuan, and the official guidance states plainly that the exceeding part is prohibited.
That is worth separating from the cash declaration above. Foreign currency over the equivalent of US$5,000 is a declaration requirement. Renminbi over 20,000 yuan is a limit. Declaring more will not make it allowed.