The invention discloses an in-water construction apparatus for a floating type water surface photovoltaic array. The in-water construction apparatus includes an installation platform, an auto-lifting system, and a dragging system. In the invention, the water surface photovoltaic array is designed by assembling a plurality of sub-arrays, wherein when a plurality of photovoltaic modules are assembled on the installation platform to form a first sub-array, the installation platform is decreased by the auto-lifting system until the first sub-array can float on the water surface independently. The dragging system drags the first sub-array to ensure that at least one row of the photovoltaic modules remains above the installation platform. The auto-lifting system then is reset, and on the basis of the last row of the photovoltaic modules, a present sub-array is installed; or after the present sub-array is installed on the installation platform, the present sub-array is jointed to the last row of the photovoltaic modules face to face; the auto-lifting system then decreases the installation platform until the jointed sub-array group can float on the water surface independently; and finally, the dragging system drags the sub-array group and ensures that at least one row of the photovoltaic modules remains above the installation platform; and repeatedly in this way, the whole water surface photovoltaic array is assembled completely, and finally the water surface photovoltaic array is dragged to destination.