In the trenching, one of the working targets of underwater construction robots, a long ditch for cable laying is dug by rotational torque of the trenching cutter generated by the force of bringing the trenching cutter into contact with the underwater ground. This contact force is generated by operating the boom cylinder which enables up and down movement of the trenching cutter or by the propulsive force of the track. This contact force applied to trenching cutter for trenching work should be maintained properly because when this force is excessive or insufficient, the trenching cutter stops or the work efficiency is not ensured, respectively. For the remote-controlled underwater construction robot, the operator should maintain the optimum torque of the trenching cutter by repeatedly operating the proportional control valve for driving the track drive motor or boom cylinder. This work requires a period of skill practice for proficient performance even on the ground and also causes fatigue quickly by repeated work even to expert. In the case of the seafloor, the work condition is more severe by the difficulty in obtaining visibility due to high concentration of float in the water caused from trenching work. The present paper proposes an active control technique for maintaining the optimal torque and speed for the trenching work. In order to evaluate the effect of the proposed control method for the trenching work, the experiments were performed.