A new method of equivalent pre-stress is suggested by used of methods of equivalent strain and temperature according to different working mechanisms between free segment and consolidated segment of pre-stress anchor cable. The mechanism on reinforcement of pre stress anchor cable is simulated by finite element. Due to enhanced toughness and anti-crack of root effect, anchor cables restrained the rock mass deforming toward surface. Rock mass around consolidated segment sustain tension. The anchor cable of unequal length should be used to avoid the anchor in arrangement of the same rock layer. The space between of anchor cables should be widened under the conditions of satisfying the need of engineering.