On-line monitoring system for MOA has been adopted in more and more applications in intelligent substations, but also encountered some problems during the application process, such as insufficient reliability , data accuracy, low integration and high rate of system failure. In order to solve the above mentioned problems, the design for the on-line monitoring system for MOA is improved. Integral structure, design of the sensor, installation method and the design of the intelligent electronic device are proposed, and a high measurement accuracy, flexible and adaptable on-line monitoring system for MOA with high reliability is provided, which also fully supports fiber-based GOOSE, SMV and station level MMS to better meet the application requirements in intelligent substations.