Underwater propeller was an important part of underwater robot. In order to improve its torque response performance, a method of direct torque control of underwater propeller based on sliding mode observer position detection was proposed. The method introduced a hyperbolic tangent function into the line back electromotive force (EMF) sliding mode observer, and this observer was used in underwater propeller direct torque control systems. Therefore, the drive control system performed sector discrimination and torque estimation through the underwater thruster line back EMF observation. Then 6 discrete commutation signals were detained. Finally, the underwater thruster direct torque control without position detection was realized. The simulation implementation showed that the proposed control method could observe the line back EMF value well and improve the torque dynamic response. The system jitter amplitude was small.