To meet the demand of static security analysis of power system， a forewarning method is proposed based on the voltage stability index related to the maximum nodal load capacity. The method uses red， yellow and green colors to label a power system capacity and boundaries to withstand load increasing at a single node. The voltage stability indices indicating the maximum nodal load capacity are defined as feature vectors. According to the real time feature vectors， nodes are classified into different sets with fuzzy clustering method and colored according to the logical rules. The permitted load increasing ranges can be obtained in off-line simulations. Simulations on an IEEE 30-bus system illustrate the effectiveness of the method.