In this paper a novel elongatable robotic snake(eSnake) based on 3-RSR mechanism is proposed. The mechanism consists of three identical kinematic chains in parallel and served as a joint connecting the body of the eSnake. It has three degrees of freedom, namely pitching, yawing and elongating, which enables spatial locomotion. Using pitching and yawing motion of eSanke’s body-joints, it can obtained the traditional serpentine movement. Furthermore, elongating of the joints improves the locomotion capability of the eSnake further by bringing two novel gaits, namely the rectilinear gait and the augmented serpentine gait, where the rectilinear helps the eSnake crawling in narrow space and the augmented serpentine gait which is indeed a coordinated motion of the serpentine gait with the rectilinear gait improves robot’s locomotion speed and energetic efficiency. In this study, we use the viscous friction model as the basis for dynamic modeling to analyze the interaction between the robot and environment, together with the kinematics and the two-dimensional morphological model of the designed gait to obtain the corresponding control equations. An eSnake consists of two segments and one body-joint was tested to verify the feasible of the mechanism. It is shown that the eSnake can achieve the expected locomotion gaits and can significantly improve the speed and efficiency of motion in augmented serpentine gait.