With the development of the ultra-high voltage（UHV）interconnected grid，increase in the volume of energy trading and optimization range of resources is beyond the power of existing tie-line power flow planning to adapt to.Besides， concentrated interconnection and long-distance transmission of large-scale wind power is making new demands on the coordination of generation and transmission scheduling.To remedy such a situation，the network flow theory is used to optimize the energy trading between regions and the regional unit-on capacity.On the basis of the present start-up modes，a coordination and optimization method for generation and transmission scheduling of power system is proposed and，with the next days energy trading power and tie-line transmission capacity between regions as constraints，and the regional unit-on capacity minimum as the objective，the corresponding network flow model is developed.