The invention discloses a Ti microalloyed medium-manganese steel medium-thickness plate for an ocean platform and a preparation method thereof. The Ti microalloyed medium-manganese steel medium-thickness plate is characterized by comprising the following chemical components of, by weight, 0.03-0.1 % of C, 4.0- 8.0 % of Mn, 0.02-0.10 % of Ti, 0.10-0.40 % of Si, less than 0. 005 % of S, less than 0.005 % of P, 0.02-0.05 % of Al, 0.10-0.40 % of Cr, 0.10-0.30 % of Ni, 0.10-0.40 % of Mo, 0.10-0.30 % of Cu, the balance Fe and other inevitable impurities. The preparation method comprises the following steps that 1, according to the Ti microalloyed medium-manganese steel medium-thickness plate for the ocean platform, alloy ingot is forged into a blank, heat preservation is carried out along with the heating of a furnace, and the heated blank is obtained; 2, multi-pass hot rolling is carried out on the heated blank to obtain a hot rolled plate, and then quenching is carried out; and 3, the quenched medium-thickness plate is put into the furnace for heating and preserving heat, the quenched medium-thickness plate is subjected to air-cooling to the room temperature to obtain the medium-manganese steel medium-thickness plate, and the structure of the medium-manganese steel medium-thickness plate is a tempered martensite and a fine inversion austenite dual-phase structure.