Mesoscale eddies regulate oceanic material transport and atmospheric current. The area around Xisha Islands is an important sink region for mesoscale eddies. We observed two cyclones during September to October 2019, and found that these two cold eddies had split from one local cold eddy. Using a Chinese Autonomous Underwater Vehicle(AUV), called ‘Sea-Whale 2000’, for field experiment, we successfully observed the fine structures at the interface of the two cyclones, which showed that two vertical layers of water developed into three layers. The AUV was operated at 300 m and captured the temperature, salinity and velocity oscillation at the bottom of the two cyclonic eddies. A simple barotropic quasi-geostrophic numerical experiment showed that the slope gradient effect played a major role in eddy splitting. Through energy conservation analysis, we confirmed that the barotropic instability was a major contributor to the development of the cold eddies.