With the continuous improvement of the pressure-bearing capacity of pressure vessels, higher requirements are put forward for the mechanical properties of materials. 40CrNi3MoV steel is a typical pressure-vessel steel, which mainly depends on carbide strengthening. To further improve its mechanical properties, Al and Cu were added to the material to form intermetallic compound precipitates for strengthening, then the precipitation behavior of NiAl and Cu and their effects on mechanical properties were studied. The size, composition, structure, and morphology of NiAl and Cu precipitates were characterized by SEM, TEM, and EDS. The distribution characteristics of precipitate-forming elements were characterized by three-dimensional atomic probe, and the mechanical properties of the experimental steel were compared. The results show that B2-NiAl precipitates coherent with the matrix are formed in the experimental steel after adding only Al. These NiAl precipitates are mainly precipitated at the grain boundaries and are large; after adding Cu, a bcc-ordered Cu-rich phase coherent with the matrix is precipitated. At this point, the large-scale precipitation of the NiAl phase at the grain boundaries is reduced and nanoscale NiAl precipitation is promoted in the crystal. The mismatch between the NiAl precipitates and the matrix lattice in the experimental steel is small, and the tensile strength increases by 200 MPa; The uniformly distributed, fine Cu-rich phase and the refined NiAl phase increase the resistance to dislocation, and the yield strength is also increased by 200 MPa. However, a large quantity of fine and high-density NiAl and Cu precipitates reduce the critical strain of cracks in the tensile process. Therefore, compared with the experimental steel with only Al added, the tensile strength is not improved.