For the cold forging process of structural steel 20MnMoB after quenching and tempering, a series of compressive tests for cylindrical specimens which were processed by different quenching and tempering technologies were conducted under different deformation degrees, and the equivalent strain, dislocation density and initial grain size of forgings were obtained respectively by finite elements simulation, Xray diffraction (XRD) and electron backscattered diffraction (EBSD). Combining with the measured values of Vickers hardness at the corresponding positions, the influences of three factors on the hardness of cold forgings were investigated and analyzed. The results show that the hardness is closely related to the three factors, and the hardness increases with the increasing of equivalent strain and dislocation density and decreases with the increasing of grain size. Thus, the specific data of hardness values can be applied to the construction of hardness prediction model.