Articulated tracked vehicles possess outstanding traveling capability due to the special articulated steering mechanism (ASM), which makes them be widely used in many application areas. In view of the importance of ASM, we design a new structure of such mechanisms. The designed 4-DOF ASM can perform yaw and pitch movement actively while roll movement passively. To work efficiently, the ASM is designed to operate in three different modes: “float” mode, “lock” mode, and “active control” mode. Then we deduce the kinematics of articulated vehicle and analyze the workspace of the front vehicle with respect to the rear vehicle to demonstrate the motion performance of the designed ASM. With the elongation and shortening of hydraulic cylinders, the maximum steer angle is beyond 0.4 rad, the range of the rear pitch angle is (-0.22, 0.27) rad, and the range of the front pitch angle is (-0.45, 0.5) rad.