Abstract: Retreat evolution of steep rock slopes is one of complex geological mechanical and dynamic geomorphology processes and is also the dominant developing mechanism for perilous rock, collapse and avalanche in mountainous areas, in keeping with the chained regularity of rock slope evolution. The control fissure in perilous rock is regarded asⅠ-type crack for a single perilous rock block. A method to calculate the strength factor of fracture stress of rock is established considering the creep rock effect by introducing the general Kelvin model. With regard to steep rock slopes with gentle dip, based on engineering mechanics, the method to calculate loads acting on perilous rocks is established. The strength factor of fracture stress and the control fissure in perilous rock considering the creep rock effect is combined with the rock tenacity, and the criterion for collapse of perilous rock blocks on steep rock slopes is proposed. Case studies show that the mechanical mechanism for the retreat evolution of steep rock slopes is consistent with the actual one.