Abstract: The purpose of this paper is to construct a predictive model for the very-high cycle fatigue limit of carburized Cr-Ni alloy steel. Very-high cycle fatigue tests were carried out on carburized Cr-Ni alloy steel under two stress ratios, and the results showed that the S-N characteristics of carburized Cr-Ni alloy steel showed a continuous downward trend. By observing the crack morphology on the fracture surface through scanning electron microscopy, it was found that surface failure was the main failure mode of the fatigue specimen when the fatigue life was less than 5×10⁵ cycles, and internal inclusion-FGA-fisheye-induced failure became the main failure mode of carburized Cr-Ni alloy steel when the life was higher than 1×10⁶ cycles. The FGA size was evaluated using the crack propagation threshold, and finally, combined with the El-Haddad model, a fatigue limit prediction model based on the threshold stress intensity factor of long cracks was constructed. Compared with the fatigue limit predicted by other models, the threshold stress model constructed had the smallest prediction error, and the predicted results were more conservative.