Heavy rainfall can easily induce serious construction disasters in the entrance sections of double-line tunnels with small net spacing. Based on on-site investigation and numerical simulation of a tunnel project in Guangxi, the instability characteristics and influencing factors of the surrounding rock at the entrance sections of the small net spacing tunnels are analyzed, according to which, the deformation control techniques for the entrance section is proposed, and the reliability of these measures are verified by on-site testing. The research results indicate that the shallow and loose surrounding rock at the entrances of the tunnels, the short excavation distance between the left and right tunnels, and heavy rainfall are the main causes of the large deformation and collapse of the surrounding rock in the project; and the deformation of the surrounding rock of the advance tunnel increases sharply with the decrease of the excavation displacement between the advance tunnel and the subsequent tunnel. Compared to the up and down step method, the seven step excavation with core soil method has significant advantages in controlling the deformation of the surrounding rock by reducing the span and implementing timely support measures. In addition, the deformation of the surrounding rock increases significantly after immersion and deterioration, and cannot be effectively controlled by increasing the excavation offset and changing the construction method. Based on these findings, a series of deformation control techniques are proposed, such as backfilling and sealing the surface collapse, increasing the excavation gap between the left and right tunnels, using the seven step excavation with core soil method for construction, shortening the excavation footage, and increasing the support strength, which are proven to produce excellent control effects.