The invention provides steel used for a steam turbine auxiliary machinery of a nuclear power unit and a manufacturing method thereof, the steel comprises the following chemical compositions: in weight percentage, 0.15%-0.25% of C; 0.15%-0.35% of Si; 0.70%-1.0% of Mn; equal to or less than 0.010% of P; equal to or less than 0.010% of S; 1.50%-2.0% of Cr; 0.001%-0.003% of Al; 0.60%-0.80% of Mo; 0.005%-0.015% of V; 0.005%-0.015% of Nb; equal to or less than 0.020% of W; and with the balance being Fe and unavoidable impurities. A continuous casting billet is rolled at a high temperature and under a high pressure, the rolling beginning temperature is equal to or more than 1050 DEG C ,the finish rolling temperature is equal to or more than 900 DEG C, the total rolling reduction is equal to or more than 70%, the continuous casting billet is naturally cooled after rolling; the normalizing temperature is 920 +- 10 DEG C with the heat preservation time of 2-4min / mm, the tempering temperature is 685 +- 10 DEG C with the heat preservation time of 5-10min / mm. The yield strength (Rel) and the tensile strength (Rm) of the steel plate are respectively equal to or more than 420N / mm<2> and equal to or more than 535N / mm<2>, after simulate post-welded heat treatment, the yield strength (Rel) and the tensile strength (Rm) of the steel plate are respectively equal to or more than 380N / mm<2> and equal to or more than 500N / mm<2>, and the heat-treatment and the simulate post-welded heat-treatment impact absorbed energy of the steel plate are between 100J and 210J.