In the construction of Nanchanggou shallow large-span tunnel which passed through the road for the passenger dedicated line of Chengdu to Guiyang, various heavy vehicles posed threat to the tunnel construction and road traffic safety. FLAC3D was used for numerical stimulation of construction progress under the conditions of no traffic and with traffic load respectively to analyze the stress and deformation under the different working conditions. The results showed that the traffic load had little effect on the tunnel construction. The analysis on the tunnel passing process and the monitoring data show that the numerical stimulation of under-passing Nanchanggou tunnel is reasonable, which provides useful guidance for construction.