Traditional UFLS and UVLS operate independently using either frequency or voltage measurements without considering the actual voltage and frequency dynamics interaction under various system conditions， load characteristics or contingency scenarios. In some cases， it may cause severe under-control or over-control. This paper presents a new method of automatic load shedding control integrating voltage and frequency dynamics interaction in the device's real-time control algorithm. The key techniques are to measure the voltage， frequency and active load demand at the device installation bus， and to calculate a synthesized state variable for triggering stepwise load shedding operation by taking into account a voltage-frequency interaction under the concerned system operating condition， load characteristics or contingency scenarios. Time domain simulation results of a real power system show that the new method has better adaptability to system operating conditions， load characteristics and contingency scenarios.