A method is proposed to analyze the short-circuit current over-limited mechanism for 500 kV power systems using the self-impedance at the short-circuit point. The method can be applied to assess the adaptability of short-circuit current limiting measures. Based on two-port network theory， an equivalent circuit with only two nodes is formed， using which the main influencing factors in short-circuit currents of different locations and fault types from the physical meaning of self-impedance composition at the short-circuit point are analyzed， and the over-limited mechanism is revealed. As is indicated， the short-circuit current at the 500 kV side mostly depends on the grid structures at the 500 kV side， correspondingly the limiting measure is mainly to change the parameters of the 500 kV power grid. The short-circuit current at the 220 kV side is affected not only by the equivalent impedance of transformers but also by the grid structures at the 220 kV side， and the sensitivities of the two aspects vary at different substations. According to the various sensitivities， the limiting measure should be adapted to increase the equivalent impedance of transformers or to change the parameters of the 220 kV side. The proposed method has been applied to the short-circuit current analysis and the limiting measures assessment of Guangdong 500 kV power grid. It is proved that the method is valid and with high practicability.