The present invention relates to an underwater flexible unit used on an oceanic drilling riser. The flexible unit is mainly composed of a main structure and a flexible element assembly; wherein, the main structure is connected with the other parts of the riser and a lower annular blowout preventer; the flexible element assembly, which is positioned in the main structure and retained by the main structure, is composed of an upper spindle, a flexible element and a base; and the flexible element is connected with the upper spindle and the base and fixed in a body by a fixing flange, bolts and nuts. Adopting the rotational stiffness of the flexible element connection structure, the flexible unit is more effective than the spherical steel connection in terms of the control of the angle of the riser, and can be adapted to rotation in any direction, and the shock-absorbing and damping effects of the flexible unit are better than the shock-absorbing and damping effects of the spherical steel connection. The flexible unit can be flexibly designed in order to meet different oceanic drilling requirements. The flexible unit has the following advantages: the whole main structure has strong bearing capability, the structure is simple and compact, and maintenance and operation are convenient. Flexible units with different specifications can be designed according to different oceanic drilling requirements.