The invention relates to a stress-strain testing system for a large-diameter steel pipe pile of an offshore wind turbine and a construction method and aims at providing the stress-strain testing system for the large-diameter steel pipe pile of the offshore wind turbine and the construction method to effectively test the deformation and stress of the large-diameter steel pipe pile so as to solve the problems that in the existing testing technology, the bearing load is small, and the anti-corrosion standard is low. According to the technical scheme, the stress-strain testing system for the large-diameter steel pipe pile of the offshore wind turbine is provided with the steel pipe pile, and copper strip type sensing optical cables are correspondingly welded to the two side faces of the steel pipe pile in the axis direction and each sequentially covered with an epoxy bonding agent, gold foil paper and angle iron welded to the steel pipe pile with the corresponding copper strip type sensing optical cable as the center; fiber cores of the copper strip type sensing optical cables are subjected to through connection to form high-strength armored optical cables through special clamps and then are guided out; and the high-strength armored optical cables are in continuous connection with a Brillouin optical fiber interrogator. The stress-strain testing system and the construction method are suitable for the technical field of geotechnical engineering testing and detecting.