Before the Stay Duration of your Chinese visa is expired, you need apply for Chinese Visa extension in the Exit & Entry Department under the local Public Security Bureau.
You need provide the following requirements:
1. Your valid passport and Chinese Visa.
2. Provide related certificates and answer some questions. For example, you need be in hospital in China, you should provide the certificate from the hospital.
3. Fill in a Visa, Certificates Extension Application Form for Aliens
4. Pay for Visa Fees.