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The small clear distance tunnel formed by the combination of the newly built four-lane tunnel and the in-situ extension of the existing tunnel has become one of the important types of reconstruction and expansion of the tunnel. The reasonable clear distance of the small clear tunnel is the difficulty in design and construction. In order to fully optimize the net distance of the super-large section and small-distance tunnel, finite difference software was used to study the influence of the construction net-distance in the reconstruction and expansion of the super-large section and small-distance tunnel, and the distribution of the plastic zone in the middle rock under different net distances. At the same time, using the penetration of the plastic zone in the middle rock as the criterion, the conclusion is drawn that the reasonable clear distance of the Shenshanxi Tunnel is 1.25B. In addition, the deformation characteristics of the surrounding rock during the construction of the existing tunnel to the new tunnel are also analyzed. The results show that: (1) With the increase of the tunnel clear distance, the smaller the change of the vault settlement and invert heave value, the smaller the final cumulative settlement. From the perspective of displacement, the reasonable clear distance between the existing tunnel and the newly built tunnel should be controlled above 1.25B.
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