Abstract: With the upgrading of the automotive industry, the fatigue resistance of frame steel, as the core load-bearing component of vehicle frames, has become a key focus of technological research. In this paper, low-cycle strain fatigue research was carried out on two typical low-alloy frame steels, 420L and 610L. Strain-controlled fatigue tests were adopted to simulate the large-strain working conditions of vehicles under frequent start-stop and complex road conditions. Combined with universal testing machine detection, scanning electron microscope（SEM） observation and metallographic microscope analysis, the fatigue characteristics of the two steels were systematically investigated. The results show that 420L steel has a ferrite-pearlite structure with coarse grains, while 610L steel has a bainite structure with fine grains, and its yield strength（584 MPa） and tensile strength（643 MPa） are significantly higher than those of 420L steel（360 MPa, 455 MPa）. The fitting of fatigue test data by the Manson-Coffin formula shows that the fatigue ductility coefficient of 420L steel（0.424） is higher than that of 610L steel（0.192）, but its fatigue life intersection point（N_c=3.4×10～4 cycles） is significantly larger than that of 610L steel（N_c=6.2×10～3 cycles）, indicating that 420L steel has a high proportion of plastic strain amplitude and a higher risk of fatigue failure. Fracture analysis found that the striation spacing in the propagation zone of 420L steel is uneven with secondary cracks, while the striations in the propagation zone of 610L steel are fine and uniform, which confirms that the fine-grained bainite structure of 610L steel can effectively inhibit crack propagation and improve fatigue stability. This study can provide a theoretical basis for the composition design and process optimization of frame steel.