The utility model relates to a robot mechanism, in particular to an underwater-gliding snake-like robot. The underwater-gliding snake-like robot comprises multiple modules, each of which comprises a self-expansive portion and a rotary driving portion. Each self-expansive portion comprises an expansive housing, an elastic rubber sleeve, an expansive steering engine, a steering engine output shaft and connecting-rod telescopic mechanisms. Each expansive steering engine is installed in each expansive housing. Each steering engine output shaft is connected with each expansive steering engine. The multiple connecting-rod telescopic mechanisms are evenly distributed in a peripheral direction of each steering engine output shaft. Each elastic rubber sleeve is sleeved outside each expansive housing and connected with each expansive housing in a sealing manner. Each expansive steering engine drives each elastic rubber sleeve to expand or contract via the connecting-rod telescopic mechanisms in order to increase or decrease the size of each self-expansive portion.The underwater-gliding snake-like robot has following beneficial effects: perfect combination between long voyage and motility of the underwater-gliding snake-like robot is achieved; each self-expansive portion helps to achieve underwater lower-power gliding of the underwater-gliding snake-like robot; and underwater flexibility and mobility of the robot is realized by a snake-like structure.