Abstract: The microstructure and property of the newly-developed DT345 pre-hardening plastic die steel after different heat treatment processes was studied. The result shows that if on-line pre-hardening inlet temperature is at 850℃ to 880℃, and make outlet temperature is less or equal 200℃ and tempersthrough 500℃ to 530℃, which can make the pre-hardening hardness reaches at 40 HRC to 43 HRC,strengh value achieve 1 140 MPa to1 270 MPa and impact value reach 260 J to 270 J. The new product can meet the user's requirement with high-hardness and all-round performance objective.