According to the requirements of integrated monitoring and maintenance for optical transport network, the on-line monitoring system for optical cable was designed in the paper. The technology such as modularized structure design,function integration and multi-purpose communication interface were used. The switching of the voice and signaling channel was resolved. The system could monitor and control over 100 modules simultaneously. The end offices could on-line monitor optical cable and optical terminal,the functions such as the system real-time monitoring,line auto-management and maintenance for optical transport equipments were realized.