The invention discloses an annular single-drive underwater robot. According to the invention, a first steering ring, a second steering ring and a third steering ring are sequentially arranged from inside to outside in a surrounding mode; the first steering ring is connected with the second steering ring through a first rotating shaft and is driven by a first steering engine to rotate; the second steering ring is connected with the third steering ring through a second rotating shaft and is driven by a second steering engine to rotate; a driving blade device is arranged in the ring of the firststeering ring, and adjustment of the driving direction of the driving blade device in a three-dimensional space is achieved through mutual rotation among the first steering ring, the second steering ring and the third steering ring; and the robot also comprises a controller connected with the first steering engine, the second steering engine and the driving blade device, and comprises a power supply. The robot has the advantages that the three steering rings are mutually connected and can rotate mutually; the driving paddles are arranged in the ring of the steering ring on the innermost side,so single-blade direction control and driving of the robot in water are achieved; operation is quite flexible; and the overall degree of freedom is quite high.