Under frequency load shedding is widely used under emergency situations in the power system. It significantly ensures frequency stability of power system by dropping excessive loads at certain places. In order to improve the traditional UFLS schemes, the rate of frequency change is taken into consideration when calculating the quantity of active power imbalance in the power system. The measure factors of frequency change rate are analyzed and a new function is created to calculate the active power imbalance based on frequency and voltage. Moreover, a new method which utilizes frequency and voltage sensitivity to determine the location and corresponding quantity of load shedding is proposed. Voltage stability should be maintained at the time of recovering system frequency stability. The simulation results based on IEEE39 buses system show that the new self-adaptive scheme of load shedding is more reasonable and superior.