The invention relates to an offshore wind power generation mono-pile foundation horizontal bearing capacity test counterforce device and a construction method thereof. The offshore wind power generation mono-pile foundation horizontal bearing capacity test counterforce device and the construction method thereof are simple in operation, obvious in effects, low in requirements on offshore testing equipment and capable of saving the cost. According to the technical scheme, the offshore wind power generation mono-pile foundation horizontal bearing capacity test counterforce device comprises a test pile driven into the seabed; the head portion of the test pile is provided with a lower flange; a counterforce pile is obliquely driven into the seabed near to the test pile, and the upper end of the counterforce pile is provided with a hydraulic jack system; a steel box girder module is connected between the upper end of the test pile and the upper end of the counterforce pile and composed of a horizontal beam, a vertical anchoring section and an upper flange, wherein the horizontal beam is a section of variable-diameter rectangular cavity, one end of the horizontal beam penetrates the circumferential surface of the vertical anchoring section with the diameter identical to that of the head portion of the test pile, the vertical anchoring section is provided with the upper flange, and the other end of the horizontal beam is a free end abutting against a hydraulic jack.