The invention belongs to the field of steel and iron material preparation, and relates to a 56kg-grade low-yield-ratio ultrahigh-strength maritime work steel plate and a preparation method thereof. According to the component design, the steel comprises the following components of 0.05%-0.012% of C, 0.20%-0.50% of Si, 1.00%-1.50% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S, 0.010%-0.030% of Nb, 0.010%-0.030% of Ti, 0.20%-0.45% of Cr, 0.010%-0.035% of V, 0.20%-0.50% of Cu, 0.10%-0.25% of Mo, 0.50%-0.95% of Ni, 0.015%-0.045% of Als, smaller than or equal to 0.004% of N and the balance Fe and inevitable impurities, the carbon equivalent CeV is smaller than or equal to 0.55, and the carbon equivalent Pcm is smaller than or equal to 0.25. According to the method, Nb, V and Ti multi-element microalloying elements are utilized, Mo, Ni and other refined and precipitated second-phase particles are added, the refined grain strengthening and precipitation strengthening effects are improved, and the subcritical quenching and high-temperature tempering processes are combined, so that ReH of the steel plate is larger than or equal to 600 Mpa, Rm ranges from 730 Mpa to 800 Mpa, the yield ratio is smaller than or equal to 0.82, and transverse and longitudinal impact energy at the temperature of-60 DEG C is larger than or equal to 200 J; and meanwhile, formed second-phase particles are dispersed and distributed in a welding heat affected zone, the strength of a welding joint is improved, 150-220 kJ/cm input can be achieved, and the impact energy at the temperature of minus 60 DEG C after welding is larger than or equal to 100 J.