In recent years, with the continuous development of the national transportation network, the proportion of new construction projects spanning existing tunnels has gradually increased. Therefore, it is crucial to study reasonable excavation methods. This paper takes the construction project of the existing railway tunnel above the roadbed in southwest China as an example to study the impact of roadbed blasting and excavation on the existing railway tunnel through numerical simulation. Combined with the vibration speed value of each part of the lining, according to the blasting vibration speed control standard, the research results are obtained: When excavation of each layer of roadbed is completed, the vibration speed of the existing tunnel first increases and then it gradually decays with the influence of damping during the period; as the roadbed blasting excavation is gradually excavated from the upper layer to the lower layer, the overall vibration speed of the tunnel continues to increase; the maximum vibration speed appears at the top of the arch, reaching 1.403 cm/s, which is less than the control standard, and three shock absorption measures are proposed to ensure the safety of existing tunnel operations. The research results show that the blasting excavation method and corresponding shock absorption measures proposed in this article can reduce the impact of roadbed excavation on existing tunnels, with a view to providing a reasonable reference for similar cases of roadbed over-span existing tunnel projects.