Abstract: Rock cells formed by the effects of different weathering of lithology of cliffs or steep slopes are the cause for collapse of perilous rock. The mechanism of massive failure must consider the interaction of perilous rock. Based on perilous rock mechanics, the interaction of perilous rock is estimated by rotation angle of the rock. First, the rotation angle without consideration of the interation of perilous rock (considering equivalent) is calculated. Then, the rotation angle with consideration of the interation of perilous rock by the law of conservation of energy is acquired. Finally, the formula to calculate the equivalent bending moment of rock by rotation angle is obtained. The formula to estimate the time for rock collapse based on the principle of fracture mechanics and damage mechanics is also established.