Experimental tests and molecular dynamics simulations were applied to investigate the mechanism of Ni concentration on the toughness and hydrogen embrittlement resistance of X80 steel. The tensile toughness, impact toughness, and hydrogen embrittlement resistance of X80 steel increase when Ni < 1%, followed by decreases when Ni > 1%. Especially, when the Ni concentration exceeds 3%, the hydrogen embrittlement resistance of the X80 steel is lower than that of the Ni-free specimen. The relationships between free surface energy, stacking fault energy, and unilateral passivation crack growth with Ni concentration were investigated by molecular dynamics simulations. The results prove that the increase of Ni concentration can simultaneously reduce the free surface energy and stacking fault energy of Fe-Ni alloys. And Ni atoms have a more significant effect on the reduction of stacking fault energy, so that the system releases energy through plastic deformation and inhibits the generation of cracks. However, as the Ni concentration exceeds 1%, martensite and carbides begin to form on the grain boundaries. The toughness and hydrogen embrittlement resistance of X80 steel decrease with the Ni concentration.