An overall horizontal preloading-boarding system for an offshore wind turbine and a preloading-boarding method. The system comprises: self-propelled modular carrier vehicles (20) disposed on a wharf, the carrier vehicles comprising tower carrier vehicles (24) and nacelle carrier vehicles (25), wherein the tower carrier vehicles comprise lower tower carrier vehicles (241), middle tower carrier vehicles (242), and upper tower carrier vehicles (243), a lower tower (11), a middle tower (12), an upper tower (13), and a nacelle (14) of a wind turbine (10) are correspondingly mounted on the three types of carrier vehicles, and the lower tower carrier vehicles (241), the middle tower carrier vehicles (242), the upper tower carrier vehicles (243), and the nacelle carrier vehicles (25) move along a same axis and implement butt joint between different sections of the wind turbine; a wind turbine installation vessel (30); and a height adjustment apparatus (40), comprising a midship support (41) vertically mounted on the deck, the midship support (41) being axially provided along a first rail (411), the first rail (411) being provided with a sliding wind turbine height adjuster (43), and the sliding wind turbine height adjuster (43) being provided with fixing clamps (433). The system uses self-propelled modular carrier vehicles on a wharf to implement butt joint of sections of a wind turbine (10) and overall preloading and boarding, so that assembly difficulty can be lowered, risks in wind turbine assembly are reduced, the preloading and boarding efficiency is improved, and energy consumption is lowered.