Submarine cable communication, as an important approach of contemporary international communication, transfers more than 95% of international communication services including internet, voice, and multinational private line services. It is the main carrier of international information, and the monitoring of its operation status is of great significance. Next-generation submarine cable systems shall realize loss monitoring as well as provide early warnings of damage events. Therefore, an coherent optical time domain reflectometry submarine cable (C-OTDR) online monitoring system integrated with phase-sensitive OTDR (Φ-OTDR) is designed to achieve the synchronous monitoring of vibration and loss over a 127-km span. In order to find a solution to the conflict between Φ-OTDR and C-OTDR on the polarization state problem, an optimal selection algorithm of polarization diversity amplitude is proposed. The proposed algorithm can effectively suppress the influence of fading noise on the sensing system and ensure the reconstruction accuracy of external disturbance events. Simulation tests on water flow impact and anchor damage dragging are conducted to verify the effectiveness of the enhanced C-OTDR system.