A granite tunnel was excavated by smooth blasting. In order to address the issues of uneven blasting contour and serious over and under excavation, and explore the influence of radial decoupling coefficient on over and under excavation, this paper establishes a three-dimensional model when the radial decoupling coefficient K is 1, 1.25, 1.5, 1.75 and 2 by using ANSYS/LS-DYNA finite element analysis software. By comparing the section damage at the bottom of the blast hole under five conditions of decoupling coefficient and calculating the average overbreak value under each condition, it is concluded that the penetration capacity between the blast holes decreases gradually with the increase of radial decoupling coefficient. When the decoupling coefficient k = 1, 1.25 and 1.5, the damage area at the bottom of the blast hole penetrates and overbreak occurs; When the uncoupling coefficient k = 1.75 and k = 2, the damage area is not connected, so it is difficult to form the smooth blasting contour. In general, when the uncoupling coefficient k = 1.5, the over and under excavation can be effectively controlled to achieve an optimal smooth blasting effect.