Applicants for U.S. immigrant visas and K-1 (fiancée) visas are required to submit a set of civil documents in support of their application. This includes, for example, birth, marriage, and police certificates.
Effective June 15, 2021, the U.S. State Department has extended the validity of police certificates to 2 years. See 9 FAM 504.4-4(A) and NVC Immigrant Visa Process–Step 7. Collect Civil Documents.
For an applicant age 16 or above, police certificates must be submitted from (1) the country of nationality, if you have lived there for more than 6 months at any time in your life; (2) the country of current residence, if you have lived there for more than 6 months; (3) other countries, areas, or localities where you have resided for one year or more, if you were 16 years or older at the time you lived there; and (4) any place where ever arrested.
No U.S. police certificate is required.
Police certificates now expire after two years, unless the certificate was issued from your country of previous residence and you have not returned there since the police certificate was issued. The visa cannot be valid beyond a certificate’s expiration date.
For basic information about how to apply for a police certificate, see the State Department’s Reciprocity and Civil Documents by Country. And see here for detailed information about China police certificates.