Abstract: A subsea coiled tubing drilling system and a subsea drilling device thereof are provided. The subsea drilling device includes an underwater derrick, a well head device and a dual-roller coiled tubing device, the dual-roller coiled tubing device includes an injection head, a roller for drilling and a roller for running casing, the injection head is arranged right above the well head device, the roller for drilling and the roller for running casing are distributed on two sides of the injection head; both the drilling coiled tubing and the casing running coiled tubing are capable of respectively penetrating through the injection head and the well head device in sequence. Drilling and casing running operations can be realized on the basis of not moving the underwater derrick or changing the self structure state of the underwater derrick, and operation steps can be simplified, and the work efficiency can be improved.