Smooth blasting is a commonly used blasting technique for the excavation of ultra-long mountain tunnels. This paper takes the construction of the Dongge section tunnel in Shenzhen City as the engineering background. It investigates the effect of relevant blasting parameters on the smooth blasting effect. Numerical models with different linear charge densities were established. Through analysis of the numerical simulation results, the optimal linear charge density suitable for this blasting project was determined. The results indicate that: The length of the main cracks in the smooth blasting layer on both sides of the blast holes and in the surrounding rock mass increases with the increase of linear charge density. At a linear charge density of 0.08 kg/m, the effect of crack penetration along the blast hole alignment is not good, with partial under-excavation observed. When the linear charge density is 0.12 kg/m, the crack length during the blasting process is the longest, resulting in greater damage to the surrounding rock mass. Through multiple analyses and calculations using numerical simulation, it was determined that a linear charge density of 0.10 kg/m yields better blasting effects. The outline of the smooth blasting is neater, and the half-hole rate is higher. Theoretical derivation and calculation results can provide reference for similar engineering projects.