An HT550 steel plate with ultrahigh toughness and excellent weldability, having the following components in weight percentage: C: 0.04% ∼ 0.09%; Si: less than or equal to 0.15%; Mn: 1.25% ∼ 1.55%; P: less than or equal to 0.013%; S: less than or equal to 0.003%; Cu: 0.10% ∼ 0.30%; Ni: 0.20% ∼ 0.60%; Mo: 0.05% ∼ 0.25%; Als: 0.030% ∼ 0.060%; Ti: 0.006% ∼ 0.014%; Nb: 0.015% ∼ 0.030%; N: less than or equal to 0.0050%; Ca: 0.001% ∼ 0.004%; the remaining being Fe and inevitable impurities; and simultaneously, the contents of the above-described elements meeting the following relationships: the relationship between C and Mn: the ratio Mn/C is more than or equal to 15 and less than or equal to 30; (%Si)x(%Ceq) is less than or equal to 0.050, wherein Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5; (%Si)x(%C) is less than or equal to 0.010; (%Mo)x[(%C) + 0.13(%Si)] is in the range of 0.003 ∼ 0.020; the ratio Ti/N is in the range of 2.0∼4.0; the relationship between Cu and Ni: Ni/Cu is more than or equal to 1.0; the relationship between Ca and S: the ratio Ca/S is in the range of 0.80∼3.0; the steel plate having a yield strength of 460MPa or more, a tensile strength of 550MPa∼700MPa, a yield ratio of 0.85 or less, -60 °C Charpy impact energy (a single value) of 60 J or more, and microstructures being fine ferrites plus self-tempered bainite with an average grain size of 15μm or less. C: 0.04% ∼ 0.09%; Si: less than or equal to 0.15%; Mn: 1.25% ∼ 1.55%; P: less than or equal to 0.013%; S: less than or equal to 0.003%; Cu: 0.10% ∼ 0.30%; Ni: 0.20% ∼ 0.60%; Mo: 0.05% ∼ 0.25%; Als: 0.030% ∼ 0.060%; Ti: 0.006% ∼ 0.014%; Nb: 0.015% ∼ 0.030%; N: less than or equal to 0.0050%; Ca: 0.001% ∼ 0.004%; the remaining being Fe and inevitable impurities; and simultaneously, the contents of the above-described elements meeting the following relationships: the relationship between C and Mn: the ratio Mn/C is more than or equal to 15 and less than or equal to 30; (%Si)x(%Ceq) is less than or equal to 0.050, wherein Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5; (%Si)x(%C) is less than or equal to 0.010; (%Mo)x[(%C) + 0.13(%Si)] is in the range of 0.003 ∼ 0.020; the ratio Ti/N is in the range of 2.0∼4.0; the relationship between Cu and Ni: Ni/Cu is more than or equal to 1.0; the relationship between Ca and S: the ratio Ca/S is in the range of 0.80∼3.0; the steel plate having a yield strength of 460MPa or more, a tensile strength of 550MPa∼700MPa, a yield ratio of 0.85 or less, -60 °C Charpy impact energy (a single value) of 60 J or more, and microstructures being fine ferrites plus self-tempered bainite with an average grain size of 15μm or less. the relationship between C and Mn: the ratio Mn/C is more than or equal to 15 and less than or equal to 30; (%Si)x(%Ceq) is less than or equal to 0.050, wherein Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5; (%Si)x(%C) is less than or equal to 0.010; (%Mo)x[(%C) + 0.13(%Si)] is in the range of 0.003 ∼ 0.020; the ratio Ti/N is in the range of 2.0∼4.0; the relationship between Cu and Ni: Ni/Cu is more than or equal to 1.0; the relationship between Ca and S: the ratio Ca/S is in the range of 0.80∼3.0; the steel plate having a yield strength of 460MPa or more, a tensile strength of 550MPa∼700MPa, a yield ratio of 0.85 or less, -60 °C Charpy impact energy (a single value) of 60 J or more, and microstructures being fine ferrites plus self-tempered bainite with an average grain size of 15μm or less. <EOS>