How is iCloud Photo Library service in China?
iCloud and Apple services are not blocked in China, the iPhone will be able to access iCloud Photo Library and www.icloud.com

Keep in mind that maybe the user will be prompted to enter additional information in order to login if Two-step Verification is enabled. This can include verification code sent to the device or SMS messages to the number associated with the Apple ID.

As alternative to iCloud Photo Library, OneDrive from Microsoft can be used for the same purpose that offers 15GB of space (I don't know if the Upload Camera bonus for additional 15GB is still active). The service has a dedicated app for iPhone (link).

UPDATE: On November 2015 Microsoft announced that will reduce the free space of OneDrive to 5GB and the Upload Camera bonus will also be removed. 

SECOND UPDATE: On 11 December 2015 Microsoft announced that the users can keep their 15GB storage plus the 15GB Camera bonus if they confirm using this page: https://preview.onedrive.com/bonus/

Dropbox is blocked in China and can't be used to backup your mobile phone photos.