The invention provides a method and system for improving and prewarning a static stability limit of an interconnecting line of a power network. According to the method and the system, a calculation formula of equivalent reactance of the interconnecting line and a calculation formula of equivalent impedance of three-coil transformers at two sides are determined through the interconnecting line of the power network and equivalent circuit models of the three-coil transformers at two sides of the interconnecting line, and a lot of real-time electrical quantities, measured by a WAMS, of the power network are substituted to calculate and determine the values, so that the accuracy of the equivalent reactance of the interconnecting line and the equivalent impedance of the three-coil transformers at two sides is improved; configuration information for dynamic reactive power sources of the three-coil transformers at two sides of the interconnecting line can be dynamically adjusted in a default operation mode through the determined values and an active power calculation formula of the interconnecting line according to near area load variation of the interconnecting line, so that the static stability limit of the interconnecting line of a complicated power system is significantly improved; and prewarning is carried out when the power system carries out power transmission according to a default power operation mode, so that the operation safety of the power network is improved.