Abstract: To systematically compare the applicability of double steel plate-concrete composite, steel, and prestressed concrete structures in semi-submersible Very Large Floating Structure (VLFS) modules, three structural forms of semi-submersible sub-modules were designed under controlled topside mass and principal dimensions. Through hydrostatic design, frequency-domain hydrodynamic analysis, and finite element structural calculations, a systematic comparison of their stability, motion responses, short-term and long-term predictions, and stress distributions was conducted. Results show that the double steel plate-concrete composite scheme has natural periods of heave and pitch extended by 8% and 42% respectively compared with the steel scheme, which helps avoid low-frequency swell effects. Under the extreme sea state of 100-year return period, its heave and pitch responses are slightly larger than those of the steel scheme, but its surge response is smaller. Long-term predictions indicate that its comprehensive seakeeping performance is superior to that of the steel scheme. Under wave-induced forces alone, the maximum stress at critical locations of the composite sub-module is only 60%-80% of that of pure steel and concrete ones, demonstrating better fatigue resistance. Direct construction cost estimates based on material quantities, welding, and concrete construction show that the composite scheme costs about 9.2% less than the steel scheme, but 13.5% more than the prestressed concrete scheme.