Based on the different dispatching time-levels of real-time active power dispatching system， a mathematical model of look-ahead control strategy by using ultra-short-term load forecasting is presented. In practical applications， according to the difference of regulating resources in different district power girds and the characteristics of frequent variation of units' composition， a control strategy of online hydro and thermal units' coordination is proposed as the supplement of the control strategy of control performance standard（CPS）. The strategy proposed has been implemented in the energy management system（EMS） of several large power systems， and has been proven effective in improving the control performance.