The present invention relates to an octagon FDPSO rapid construction method comprising steps of: (1) building a lower buoyancy aid and an internal function cabin of the octagon FDPSO on a berth or on a plane ground; (2) at the same time, dividing an upper buoyancy aid of the FDPSO into a plurality of functional blocks and conducting individual construction of the plurality of functional blocks on land in the peripheral of a dock; (3) launching the lower buoyancy aid after the construction thereof and dragging the whole body of the buoyancy aid within the dock, closing a dock gate, withdrawing water from the dock so as to anchor the lower buoyancy aid within the dock; (4) lifting the plurality of functional areas the construction of which has been completed to the upper part of the lower buoyancy aid through a suspension apparatus for modular assembly and folding; (5) installing and debugging relevant work equipments of each functional area; (6) opening the dock gate and filling in water so as to make the octagon FDPSO float in the ocean under seawater buoyancy effect. The synchronous construction of the upper and lower parts of the octagon FDPSO of the present invention shortens the time for building required on berth, dock or plane ground, thus achieving the purpose of rapid construction.