The present invention relates to a stress-strain testing system for a large-diameter steel pipe pile of an offshore wind turbine and a construction method, comprising a steel pipe pile, wherein copper belt type sensor cables are correspondingly welded on both sides of the steel pipe pile along an axis direction; each sensor cable is sequentially covered with an epoxy adhesive, gold foil paper and an angle steel welded on the steel pipe pile centering on the copper belt type sensor cable; a fiber core of each copper belt type sensor cable is transferred into a high-strength armored optical cable by a special fixture and then is led out; and the high-strength armored optical cable is connected with a Brillouin optical fiber demodulator. The present invention is applicable to the field of foundation engineering testing and detection technology.