Controlled drainage should be applied to deal with underground water in subsea tunnel. In order to study the change laws of subsea tunnel wall rock pressure, water pressure on primary support and internal force of steel arching in the process of construction, combining with the Xiamen Subsea Tunnel, the real time monitor of the wall rock pressure, water pressure on primary support and internal force of steel arching were carried out on site in the process of construction. The results show that the primary support has a high security coefficient in construction period. Grouting in advance and grouting back the primary support is necessary for subsea tunnel. In the circumstance of controlled drainage, by strengthening some measures such as grouting, the water pressure on primary support can be reduced to 1/3 of the full hydrostatic pore pressure. By monitoring and analyzing the water inflow on site, the water inflow controlling criteria are put forward. The water inflow in soft rock section in Xiamen Subsea Tunnel should be less than 0.25 m³/(m·d). The result of the paper were verified and applied in Xiamen Subsea Tunnel.