Abstract: In order to meet the requirements of high strength plasticity matching for high load bearing crankshaft, 34CrNiMo6 steel was selected for quenching and tempering heat treatment, and the influence of quenching temperature and tempering temperature on the mechanical properties of alloy steel was studied.The matching relationship between process, structure and properties was established, and the strengthening and toughening mechanism of the alloy steel after quenching and tempering heat treatment was revealed.It is concluded that the hardness and strength of 34CrNiMo6 steel gradually decrease with the increase of quenching temperature when tempering temperature is constant, and the elongation increases first and then decreases, reaching the peak value at 860 ℃, while the impact energy increases gradually.When the quenching temperature is constant, with the increase of tempering temperature, the hardness, tensile strength and yield strength show a trend of decreasing first before increasing and last decreasing, and the elongation and impact energy show a parabolic shape that increases first and then decreases.At 580 ℃tempering, the hardness and strength reach small peaks, the elongation obtains the maximum value, the impact energy than that of the maximum of 560 ℃decreases slightly.When the tempering temperature exceeds 600 ℃, the performance decreases sharply.Therefore, the optimal quenching and tempering process of 34CrNiMo6 steel is as follows:quenching at 860 ℃+tempering at 580 ℃.The strengthening and toughening mechanism of 34CrNiMo6 steel is related to the refinement degree of sorbite, residual austenite content and dislocation density in the microstructure during deformation.