Abstract: The paper discusses the development of an ultrasonic symmetric bending fatiguesystem on the basis of the three-point bending method, including the design ofultrasonic symmetric bending samples and the influence of processingerrors. The S-N curve of 50^\# axle steel is determined by theultrasonic symmetric bending fatigue system (20\,KHz).The results show thatthe S-N curve of 50^\# axle steel is a continuously decliningcurve. Compared to the S-N curve of 50^\# axle steel under the symmetrictension and compression loading, it is shown that in the high-cyclestage, the loading method has a significant effect on fatigue properties,and in the ultra-high-cycle stage, the loading method has no significantimpact on fatigue properties.