An integrated horizontal preassembling and loading-out system for offshore wind turbines, comprising a plurality of self-propelled modular transporters disposed at a yard, a wind turbine installation vessel, and lifting devices installed on the turbine installation vessel, wherein the self-propelled modular transporters comprise tower transporters and a nacelle transporter, the tower transporters comprise lower tower transporters, a middle tower transporter and an upper tower transporter for mounting a lower tower, a middle tower and an upper tower of a wind turbine, respectively, and the nacelle transporter is used for mounting a nacelle of the wind turbine; each self-propelled modular transporter comprises a frame and an installation seat installed on the frame, sections of a wind turbine are respectively mounted on the installation seats of the corresponding transporters (24, 25); and the lower tower transporters, the middle tower transporter, the upper tower transporter and the nacelle transporter are configured to move along the same horizontal axis to implement horizontal butting and assembly of all sections of a wind turbine, wherein the self-propelled modular transporters are configured to implement a load-out process of the integrated wind turbine from yard onto the wind turbine installation vessel; and the lifting devices are configured to complete a loading-out process of wind turbines from the self-propelled modular transporters, wherein the lifting devices comprise supporting legs and wind turbine lifters, wherein the supporting legs are vertically mounted on the deck of the wind turbine installation vessel, the supporting legs are provided with vertical first rails, the wind turbine lifters are in fit with the first rails and are capable of moving up and down along the first rails, the wind turbine lifters comprise sliders and cantilever beams, driving devices are arranged inside the sliders to drive the sliders to move up and down along the first rails, the cantilever beams are fixedly connected to the sliders, and fixing clamps are mounted at the end parts of the cantilever beams and are configured to hold the wind turbine. the self-propelled modular transporters comprise tower transporters and a nacelle transporter, the tower transporters comprise lower tower transporters, a middle tower transporter and an upper tower transporter for mounting a lower tower, a middle tower and an upper tower of a wind turbine, respectively, and the nacelle transporter is used for mounting a nacelle of the wind turbine; each self-propelled modular transporter comprises a frame and an installation seat installed on the frame, sections of a wind turbine are respectively mounted on the installation seats of the corresponding transporters (24, 25); and the lower tower transporters, the middle tower transporter, the upper tower transporter and the nacelle transporter are configured to move along the same horizontal axis to implement horizontal butting and assembly of all sections of a wind turbine, wherein the self-propelled modular transporters are configured to implement a load-out process of the integrated wind turbine from yard onto the wind turbine installation vessel; and the lifting devices are configured to complete a loading-out process of wind turbines from the self-propelled modular transporters, wherein the lifting devices comprise supporting legs and wind turbine lifters, wherein the supporting legs are vertically mounted on the deck of the wind turbine installation vessel, the supporting legs are provided with vertical first rails, the wind turbine lifters are in fit with the first rails and are capable of moving up and down along the first rails, the wind turbine lifters comprise sliders and cantilever beams, driving devices are arranged inside the sliders to drive the sliders to move up and down along the first rails, the cantilever beams are fixedly connected to the sliders, and fixing clamps are mounted at the end parts of the cantilever beams and are configured to hold the wind turbine.