The ultra-small spacing tunnel of Jiaozhou Bay seabed tunnel in Qingdao is took for example; the surrounding rock pressure features and support force are analyzed through the field measurement. Results show that the surrounding rock pressure and support force of the after-excavation tunnel are larger than others of the tunnel. This result is different from the theoretical predictions and experimental results of most of the small spacing symmetry tunnel. To alleviate these problems effectively, support parameters are adjusted opportunely; the radial support capacity of the after-excavation tunnel is enhanced, and reinforcement measures of rock pillar are made at the same time. The monitoring results show that the stability of the after-construction tunnel has been effectively controlled, which provides useful reference for the design and construction of similar tunnels with small spacing.