Based on the consideration of crack surge pressure of perilous rock under the blasting load， physical model of crack surge pressure of unstable rock was established. The formation of crack surge pressure is attributed to the initial velocity exerting to crack water by the blasting power. Through theoretical derivation formula of dynamic water impact pressure perpendicular to dominant fissure and increasing amplitude of crack pressure along the fissure direction were determined. Lastly， this paper gave the calculation expression of crack surge pressure under the blasting load. Taking the perilous rock W2-1 in Wangxia for example， action time to crack water was 0.2~0.25s by the blasting power； crack pressure changed nearly 10% by the blasting power which affected the crack surge pressure markedly.