The present application provides an underwater-wellhead feeding tool assembly and a use method thereof. The device controls a controller to add pressure to an inside of a drive cylinder through a first pipeline via a first port, and to add pressure to a cavity apace between a pressure-bearing cylinder and the drive cylinder through a first branch line; the controller controls, through a third branch line, the drive device to transmit a torque to the underwater wellhead feeding tool, and transmit, through a second branch line, an axial force to the underwater wellhead feeding tool; and then a sensing component transmits a detected stress signal and/or strain signal to a data acquisition device through a test lead, thereby testing and analyzing an effect of different force field conditions on mechanical behaviors and performance of the underwater wellhead feeding tool.