Foreigners who want to stay in China beyond what their current visa allowance need to apply for an extension to their visa with appropriate reasons. The extension application should be made at the municipal public security bureau 7 days before the visa expires. Multiple visas cannot be extended. 
 
Relevant documents needed
Foreigners who wish to apply for extension or alteration of visas or certificates need to provide the relevant documents:
1) A valid passport and visa as well as the documents regarding accommodation registration
2) A fully completed application form for extension or alternation
3) Documents and papers supporting reasons for extension or alternation
 
Note:
For people of different status, the requirements can be various.

Dependents
Holding official letters of office of students abroad、students' passports and residence permits, process it within the valid period of residence permits of students, for 90 days per extension.

Critical patients
Bring with medical certificates of critical condition provided by hospitals to apply for extension.

Foreigners with international marriage between Chinese
Chinese citizens' foreign spouses must provide marriage certificates, originals and copies of inhabitant registration cards and the I.D. cards of their Chinese spouses; the duration for the extension is 3 month and it can be extended for three times. What’s more, continuing apply for extension after stay over one year, one must submit a written application
 
Dependents of embassy officials
The dependents of embassy officials must provide the letters of embassy to apply visas.
 
Foreigners who are disallowed to extend
1) Foreigners whose duration of stay is shortened and those whose residence right are deprived
2) Individual travelers who have no sufficient traveling expenses
3) Family visitors who has no special reason for extension when their visa expires
 
General Notes
It is vital that you have the necessary documents with you when you travel to China otherwise entry will be denied. 
 
Fees for visas can vary from one country to another but details can be obtained from the web site of the Chinese Embassy in your country.
 
Overseas visa authorities are: Chinese Embassies, consulates, visa offices, the consular department of the Office of the Commissioner of the Ministry of Foreign Affairs in HKSAR and other agencies abroad authorized by the Ministry of Foreign Affairs of China.