The invention discloses an ultra-thin austenitic grained steel with ultrahigh strength, high toughness and easy welding and a manufacturing method thereof. The chemical compositions of the steel are as follows in terms of weigh percentage: 0.04 to 0.14 of C, 0.50 to 1.75 of Mn, 0.10 to 0.50 of Si, no more than 0.010 of S, no more than 0.012 of P, no more than 0.50 of Ni, no more than 1.00 of Cr, no more than 0,80 of Mo, no more than 0.10 of V, 0.10 to 0.30 of Cu, 0.010 to 0.050 of Al, 0.010 to 0.050 of Ti, no more than 0.050 of Nb, 0.0005 to 0.0020 of B, no more than 0.0090 of (O+N), no more than 0.15 of Pb+Sn+As+Sb+Bi and the balance of Fe and impurities. The equivalent carbon content CEQ (%) is no more than 0.60 and the crack sensitivity index PCM (%) is no more than 0.30. The manufacturing method comprises the following steps: low-temperature short-time tempering pretreatment, short-time heat preservation after extremely fast and integral heating to more than Ac3, then holding the temperature, and extremely fast quenching to the room temperature. The austenitic grain of the steel has the average size of less than or equal to 6.5Mum, the yield strength of more than or equal to 840MPa, the tensile strength of more than or equal to 945MPa and tough-brittle transition temperature of less than or equal to minus 60 DEG C. The method is particularly suitable for heat treatment of working pieces with thin specification.