The invention provides an HT550 steel plate with ultrahigh toughness and excellent weldability and a manufacturing method thereof. Based on a component system which is subjected to ultralow C-high Mn-Nb microalloying-ultramicro Ti treatment, Mn/C is controlled in the range from 15 to 30, (%Si)*(%Ceq) is less than or equal to 0.050, (%C)*(%Si) is less than or equal to 0.010, (%Mo)*[(%C)+0.13(%Si)] is in the range from 0.003 to 0.020, Ti/N is in the range from 2.0 to 4.0, (Cu+Ni+Mo) is alloyed, Ni/Cu is greater than or equal to 1.0, Ca treatment is performed, and the ratio of Ca/S is within the range from 0.80 to 3.00; TMCP (Thermo-Mechanical Control Process) is optimized, so that the microscopic structure of the steel plate comprises fine ferrites and self-tempered bainite; and the steel plate has an average grain size below 15 microns, yield strength greater than or equal to 460 MPa, tensile strength in the range from 550 MPa to 700 MPa, a yield ratio less than or equal to 0.85, and -60 DEG C Charpy ballistic work (single value) greater than or equal to 60 J; therefore, the steel plate is capable of bearing large thermal input welding while having even and excellent match of strong obdurability and strong plasticity; and as a result, the steel plate is especially suitable for a sea bridge structure, an ocean wind tower structure, an ocean platform structure and a hydroelectric structure.