The Tianjin branch of the China National Offshore Oil Corp. said it discovered the spill Friday, Xinhua reported.
The company said the source of the spill is an underwater pipe that was damaged by anchors of ships passing above it.
The State Oceanic Administration has sent out a plane and a monitoring vessel to assess possible damage.
In June, a ConocoPhillips China oil spill in Bohai Bay ruined local tourism and fish farming.
Following the ConocoPhillips spill, the Chinese government pledged to inspect all offshore drilling and exploration to prevent further spills.