The invention discloses a method for measuring sound velocity of step rock mass under an explosion area of an open-pit mine. The method is characterized by utilizing a first explosion hole of an explosion area of the previous step as a vibration source, and arranging vibration measuring instruments at the middle lower portion of a step slope in a horizontal explosion area of the next step of the explosion area of the previous step; obtaining surrounding rock vibration signals of vibration measurement points corresponding to the vibration measuring instruments; and enabling the distance difference between two vibration measurement points and an initial vibration source position to divide relative time difference of the vibration signals arriving at the vibration measurement points to obtain the sound velocity of the rock mass of the next step. The sound velocity of the cracked rock mass of the horizontal explosion area of the next step can be obtained through charged blast hole explosion in blast working, so that matching relation between explosives and rock can be obtained conveniently and quickly, and the method can be used for guiding fine blasting design and construction.