The invention discloses a high-strength high-ductility low-density steel plate based on twin roll continuous casting and a preparation method thereof. The steel plate is prepared from the following ingredients in percentage by mass: 0.3 to 06% of C, 13 to 20% of Mn, 5.8 to 6.8% of Al, 2.5 to 4.5% of Ni, less than 0.1% of Si, less than 0.01% of S, less than 0.01% of P, 0 to 0.07% of Ti, 0 to 0.1% of Nb, 0 to 0.1% of V and the balance of Fe; the strength of extension is 950 to 1000MPa, the ductility is 40 to 50%, and the density is 6.5 to 7.5g/cm<3>; the preparation method comprises (1) smeltingmolten steel; (2) flowing into a twin-roll continuous caster through a tundish, solidifying by passing through a roll gap and guiding out of a ribbon; (3) cooling to room temperature by the speed of50 to 100 DEG C/s, performing acid pickling and then cold rolling; (4) heating to 650 to 780 DEG C and performing heat preservation, and cooling to room temperature by the speed of 100 to 180 DEG C/s.The method provided by the invention saves a plurality of procedures, remarkably reduces the production cost, and the steel plate has both high strength and high ductility.