The system architecture， optimization model and control strategies of a plant-level automatic generation control（AGC） system in Mawan Power Plant are described in detail. It is proved that according to such performance parameters of each unit as coal consumption， regulating range and operating situation， the plant-level AGC system can automatically， reasonably and optimally distribute load to all units in the power plant so as to reduce generation and maintenance cost effectively， reduce the unit regulating frequency， improve the security and stability of units and expand the units life cycle. Not only the requirement of stability， reliability and security for power grid is satisfied， but also economic operation of power plant as well.