The single-jointed underwater robot fish includes a casing, a main board cabin, a motion control cabin and a battery cabin. The outer contour of the casing is a bionic fish shape, one end is a front casing, and the other end is a bionic fishtail structure. The front end of the main board cabin is affixed to the inner side of the front casing, a lens group, an eccentric anti-shake mechanism and a main PCB (printed circuit board) are set in the main board cabin. The motion control cabin is connected to the rear end of the main board cabin. A triaxial linkage device and a transmission device are set in the motion control cabin. The battery cabin is located below the motion control cabin.