According to the Regulations of People's Republic of China (PRC) Detention Center
Article 28 A prisoner in custody may, with the consent of the case-handling organ and the approval of the public security organ, communicate and meet with his near relatives.
Article 30 prison guards must inspect articles given to prisoners by their close relatives.
Article 31 A detention center, entrusted by the case-handling organ, may inspect the letters sent and received by criminals. Anyone who is found to obstruct investigation, prosecution or trial may be detained and transferred to the case-handling organ for handling.
Extended data
Measures for the administration of execution of punishment by criminals in detention centers
Forty-eighth with the approval of the leaders of the detention center, criminals can use the designated fixed telephone to talk to their relatives, friends and guardians; Criminals of foreign nationalities can also talk to their embassies (consulates) in China. The telephone bill is borne by the criminal himself.
Article 49 Ethnic minority criminals may meet and communicate in their own spoken and written languages. Foreign criminals can meet and communicate in their own language.
Article 51 A criminal may correspond with his relatives, friends or guardians. The detention center shall check the letters of criminals, and may detain the letters that hinder the reform of criminals.
Letters written by criminals to higher authorities and judicial organs of detention centers are not subject to inspection.
References:
Datong Municipal People's Government Network: Regulations of People's Republic of China (PRC) Detention Center.