Abstract: To evaluate the high-/low-cycle fatigue life of dissimilar steel welded joints, achieve a unified characterization, and reveal the differences in crack fracture locations and their transfer behaviors under HCF/LCF for different base metal combinations, welded joints of No. 35 forged steel/No. 20 cast steel and Q550E high-strength steel were selected as the objects. High-cycle and low-cycle fatigue tests were conducted, and the stress responses at the weld toe were extracted in combination with finite element calculations. Meanwhile, the prediction performances of various equivalent parameter methods were compared to verify the unified evaluation capability of the equivalent pseudo-structural stress method. The results indicate that high-cycle fatigue fractures all occur at the weld toe; under low-cycle fatigue, the 35/Q550E joint still fractures at the weld toe, whereas the fracture of the 20/Q550E joint transfers to the base metal. In the life evaluation based on the equivalent pseudo-structural stress method, 90.9% of the test data fall into the scatter band of 99% survival probability, and the prediction accuracy and scatter control are significantly superior to those of the equivalent structural stress method and the equivalent structural strain method. The research confirms that this method can reliably and uniformly evaluate the high-/low-cycle fatigue life of dissimilar steel welded joints, providing an important basis for engineering fatigue design, life prediction, and safety assessment.