The limited equilibrium and destroyed status of rock slope are defined based on the change of displacement vector by UDEC simulation. A new method to determine the safety threshold velocity of slope under explosion is proposed based on the definition. In addition, the safety threshold velocity of the Huangmailing Phosphorite rock slope is suggested. The obtained safety threshold velocity by the new method is in good agreement with the measurement. To determine the safety threshold velocity of rock slope by the proposed method is applicable.