Abstract: By using the assertion that the propagated wave signal mainly comes from the natural vibration of geophysical structure after blasting, a new technique is proposed for detecting underground large scale cracks and thin weak layers. This technique indicates that the local stiffness decreases, owing to the exist of the cracks or thin weak layers in the medium, which directly effects the vibration velocity and frequency of the medium. The character of cracks or thin weak layers can be detected by the signals (including the vibration velocity and frequency) received on the ground if placing the source of blasting vibration near cracks or thin weak layers. The calculations were performed by a recently developed SPH(smoothed particle hydrodynamics) method and shown the contrast of the blasting vibration signals between the situation when cracks were horizontal and inclined. The results drawn a conclusion and showed the possibility of this method.