Based on the mechanism of smooth blasting and the theory of plane elastic wave,the interactions of explosion stress wave and explosive gas with geological discontinuities in various orientations were analyzed and the reason that geological discontinuities influence smooth blasting effect was revealed.When the intersecting line of geological discontinuities and predetermined smooth plane is vertical to the connecting line of the centres of two blasting holes,the resistance function of geological discontinuities will weak the superposition of explosion stress wave on the connecting line of the centres of blasting holes,also weak the superposition of the quasi-static pressure of explosive gas on the connecting line,so the predetermined smooth plane is difficult to form.When the intersecting line parallels to the connecting line,that is to say,when the strike of geological discontinuities is in accord with the direction of the connecting line of the centres of blasting holes,the geological discontinuities have the function of free surface,it is also difficult to control the effect of smooth blasting.