The invention discloses a subsea device comprising a can, a string of risers, a blowout preventer and a string of tie-back pipes. The blowout preventer is installed on the can. The top of risers is connected to a rig and its bottom is mounted on the blowout preventer. The upper of tie-back pipes is fixed to the can. The lower of tie-back pipes is mounted to a wellhead in the seabed by a subsea wellhead apparatus and connected to the casings for collecting oil/gas stored in formations by the subsea wellhead apparatus. A passage communicating the blowout preventer with tie-back pipe is jointed on the can. A pipeline, transporting oil/gas, mud and cuttings in drilling process, is divided into two sections, both tie-back pipes and casings, by the wellhead apparatus settled on seabed, thereby ensuring security of the rig in the drilling process and stronger operability of entire production system.