Hello Garbo - The affidavit of non-marraige to prove that you are single is called a “certification of marriageability".
According to the American Embassy in China, this document can be prepared at the Embassy on the basis of an affidavit in which the American citizen swears or affirms before a Consul that he or she is currently legally eligible to marry. Upon receipt of an application to register a marriage, the civil affairs office will ascertain that both parties are of minimum marriageability age (generally 22 for men and 20 for women, although a higher minimum may be established by the local civil affairs office) and that both parties are single and otherwise free to marry.
Persons who have been married previously will be asked to submit original or certified copies of final divorce or annulment decrees, or of death certificates if widowed.
The American partner to a marriage in China will generally be asked to submit a “certification of marriageability" which can be prepared at the Embassy on the basis of an affidavit in which the American citizen swears or affirms before a Consul that he or she is currently legally eligible to marry (persons who have previously been married need to show a certified divorce decree, annulment decree, or death certificate both to the Embassy when preparing this certificate and to local authorities.
You should take a good photocopy with you when you go to register the marriage. Generally, if you present the certified copy with the copy for their review, the Chinese authorities will accept the copy. This is also true for your spouse if he or she has previously been married.
NOTE: Translation of documents usually takes about a month, but can be completed within 10 days at double the original cost. Translations should be obtained from and certified by one of the Municipal Public Notary offices.
(Some of this info was lifted from a 3rd party site).
Edited by Eli on 3/4/2010 at 12:02 PM EST