This paper presents a novel underwater horizontal directional drilling platform which can be remotely operated in sub sea environment by using a submersible coiled tubing system. The submersible reel on which 150 meters of 2′′coiled tubing are spooled can release or retrieve with the submersible injector simultaneously, that allows a horizontal distance of 100 meters in maximum for shallow horizontal drilling. The underwater mud jetting system consisting of mud pump, flexible mud containers and mud-seawater mixture equipment can provide continuous mud supply for the drilling tools. For the underwater drilling navigation, the IMU-7200 inertial measurement unit with high precision is applied to calculate the actual drilling trajectory. And the drilling parameter can be regulated in real-time according to the position and azimuth of the drill head to complete whole drilling and back-towing tasks. As all control systems working in a remotely operated manner, the complete underwater power units and plentiful underwater sensors are also carried on the platform systematically to facilitate drilling operations. This underwater horizontal directional drilling platform can be widely used in salvage operations, marine geology explorations, as well as some underwater installations.