Power flow forecasting is a fundamental function in early warning and security countermeasure system（EWSC） for power grid. The ultra-short term and daily power flow forecasting methods are both presented. The factors affecting robustness and accuracy of the results of these two methods are analyzed. Power flow forecasting software is introduced which has been imbedded in a EWSC system. The recorded results from a provincial power grid are given and the practicality of the proposed method is validated.