The invention provides a novel rope sensor in underwater two-dimensional situation utilizing the constant tension principle of torque motors. The rope sensor comprises two parts of a velocity-measuring mechanism and an inner control circuit, wherein, the velocity-measuring mechanism comprises a support, a case arranged at one end of the support, a reel and a velocity-measuring torque motor which are arranged in the case and share the same shaft. The rope on the reel is connected directly with the object under test. The inner control circuit is arranged in the case and electrically connected with the velocity-measuring torque motor and an outside super-computer controller respectively. The invention can be adapted to underwater moving object velocity-measuring environment and has the advantages of simple structure, easy mounting, high cost performance, durability, practicability, suitability for general scientific studies and high expandability.