The invention discloses a basic module of an extra-large marine floating structure. The basic module is composed of a lower-portion floating body structure, an upper-portion structure and middle connecting structures. The lower-portion floating body structure comprises multiple dispersedly arranged floating bodies and is wholly in an extra-large water plane area form, the sum of the water discharging areas of the floating bodies is greater than the equivalent-quantity water volume of the whole weight of the floating structure, the upper-portion structure is a frame structure or a box body structure, the middle connecting structures are dispersedly arranged in space, and the lower-portion floating body structure is connected with the upper-portion structure through the middle connecting structures to form a hyperstatic combined space structure. Two basic modules can form the kilometer-level extra-large marine floating structure through one-time splicing. The basic module has very strong wave exciting load resistance and load change capability, has very strong sway-resistant stabilizing rigidity and can remarkably decrease relative motion during basic module splicing and connector load after splicing.