A high-manganese non-magnetic steel containing niobium and a preparation method thereof belong to the technical field of non-magnetic steel production. Components of the high-manganese non-magnetic steel containing niobium comprises, by mass percent, 10 to 15 % of Mn, 0.8 to 1.2 % of C, 0.3 to 0.5 % of Si, less than 0.008 % of P, less than 0.005 % of S, 0.01 to 0.02 % of Nb, and the balance being Fe. The preparation method comprises steps of smelting, forging, hot rolling, water toughening, and water quenching after the hot-rolled plate is kept in a heating furnace for 15 minutes at 1000 DEG C. The method is advantageous in that the high manganese steel is pre-deformed by using a rolling method, thereby being capable of refining grain size and improving organization and performance, and a finished product has tensile strength of 900 to 1100 MPa, elongation percentage of 50 to 60%, Rockwell hardness of 15 to 22 HRC and magnetic permeability of less than 1.0002. According to the invention, the non-magnetic steel is superior in magnetic, mechanical and processing properties, and the method is simple in production, low in economic cost, and is applicable in the fields of mechanical production and transportation, such as nonmagnetic structural members and nonmagnetic molds used for magnetic shielding in large and medium-sized transformer oil tanks, train tracks, etc.