The microstructure and mechanical properties of the longitudinally profiled EH40 steel plate with the specification of (30~50) mm×2600 mm×3000 mm were investigated by means of tensile test, impact test, hardness test, optical microscope and TEM. Results show that the microstructure and mechanical properties of the longitudinally profiled steel plate is diverse due to the experienced different processes of the thin end and thick end. As the thickness of the steel plate increases the yield strength decreases from 534 MPa to 489 MPa, while the tensile strength decreases from 599 MPa to 569 MPa. When the impact temperature is -60^oC, the absorb energy is over 200 J for the thin end of steel plate, while the absorb energy fluctuates for the thick end. The grains of the thick end with lower bainite content are larger than those of the thin end. There is Bainite within the full cross section for where with the plates of 30 mm and 40 mm in thickness. However, all the bainite disappeared for where with the plates of 8 mm and 50 mm in thickness. The precipitated phases of both the thin ends and thick ends are (Nb, Ti)C. For the plate at the thin ends, there exists large amount of precipitated phases with small particle size, whereas small amount of precipitated phases with large size for that at the thick ends.