The invention relates to a method for monitoring the thermomechanical behaviour of a subsea pipe (2) carrying pressurised fluids and produced by assembling unitary pipe elements (4), the method comprising a step of determining a mechanical signature specific to each unitary pipe element, consisting in using a measuring cable (16) provided with a fibre optic sensor using Brillouin backscattering to measure the deformations in each unitary pipe element when it is subjected, while ashore, to different mechanical stresses of predetermined directions and intensities, and to establish a stiffness matrix associated with the mechanical signature of each unitary pipe element; a step of determining a thermal signature specific to each unitary pipe element by measuring the temperature changes in said unitary pipe element when it is subjected, while ashore, to different electrical heating power values, and establishing a thermal transfer function associated with said thermal signature of each unitary pipe element; and a monitoring step consisting in recovering the variations of the optical signal injected in the sensors when the pipe is in service, and determining the changes in the mechanical and thermal signatures of each unitary pipe element.