A series of problems, including large deformation of supporting structures, shotcrete peeling, and yielding of the steel frame, occurs constantly during tunnel construction in soft clay layers. These problems considerably raise the risk of tunnel construction. Thus, the deformation characteristics of soft clay around the tunnel during construction must be investigated to guarantee safety. In this research, a field test was conducted on 50 sections of a tunnel, which was built in high-plasticity clay layers. Then, the deformation characteristics of surrounding rocks and influencing laws caused by burial depth, invasion thickness of soft clay, and atmospheric precipitation were discussed. Results indicated that surrounding rocks are most likely to undergo large deformation during tunnel construction when the burial depth of tunnel ranges from 1.5D to 2.5D (D is the tunnel excavation span). Tunnel deformation also increases rapidly when the invasion thickness exceeds 60% of the tunnel height. The ratio between clay thickness and burial depth of the tunnel is another crucial index that could cause a large tunnel deformation as it exceeds 0.25. In addition, a significant correlation was observed between tunnel deformation and rainfall during the construction period. The deformation of surrounding rocks increases rapidly with rainfall and will continually develop for 1-2 weeks when the average daily rainfall is greater than 4 mm.