Nuclear power pump shaft, as a crucial load-bearing component experiences corrosion failure due to long-term exposure to cooling water, this corrosion failure is a critical factor limiting its service life. For such corrosion, sodium phosphate, as an important corrosion inhibitor, herein, of which the inhibition performance on the 40Cr steel was studied by using potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometer. The results indicate that the corrosion rate decreases significantly with increasing sodium phosphate (Na₃PO₄) concentration. At lower concentrations (1 × 10^-4 and 1 × 10^-3 mol/L), corrosion still occurs, forming loose and porous corrosion products composed mainly of Fe₃O₄, Fe₂O_3, and γ-FeOOH, which provide poor corrosion protection. At a concentration of 1 × 10^-3 mol/L, a more stable α-FeOOH phase is emerged, with volume fraction ratio of γ-FeOOH to α-FeOOH being 39.65% and 60.35%. The compact corrosion products of α-FeOOH and Fe₃O₄ effectively decelerate the corrosion process and improve the protective properties of the rust layer. When the concentration is increased to 1 × 10^-2 mol/L, a dense and stable passive film forms on the 40Cr steel surface, primarily consisting of FePO₄ and Fe₂O₃, this film effectively isolates the substrate from the corrosive medium, maintaining the steel in a passive state and thereby significantly inhibiting the corrosion process.