Abstract: The disclosure belongs to the technical field of underwater operations, in particular relates to an operation system and an operation method for underwater jet trenching and cable laying. The underwater walking device is provided at a lower end of the main body; the two jetting arms is located at a front end of the main body; the jetting suction device is arranged at a rear end of the main body and includes a suction pipeline and a second jetting pipe, the suction pipeline is configured to suck sediment in the trench through the suction port before a cable settles to the bottom of the trench and to discharge the sediment to outside of the trench through the discharge port; the second jetting pipe is configured to spray a horizontal jet to scour an inner side wall and the bottom of the trench along the length direction of the trench.