Reasonable selection of low cycle fatigue life assessment models is of great significance for improving the accuracy of gear shaft fatigue life prediction. The input shaft of hybrid transmission was taken as the research object, and the finite element model of the input shaft with a hardened layer was established, and the nonlinear mechanical properties of the model was defined. The fatigue load under the torsional vibration state of the transmission system was applied to calculate the stress and strain of the input shaft, and the weak fatigue life position of the input shaft was determined based on plastic strain. Then, 5 typical low cycle fatigue life assessment models were combined with 8 assessment model fatigue parameter estimation methods, and the critical plane method was applied to calculate the fatigue life. Finally, the bench test was conducted to verify the accuracy of the fatigue life assessment models, and a fatigue life assessment model suitable for gear shafts was determined. The results show that the predicted initial cracking positions of different fatigue life assessment models are all at the locations with higher plastic strain, and the prediction is consistent with the experiment. The fatigue life of the input shaft calculated by different fatigue life assessment models is smaller than the test life. Among the models, the fatigue parameters estimated by the median method are substituted into the SWT model to obtain a good agreement with the test life, with an accuracy within 2.5 times the life line, which meets engineering needs. SWT model can guide the design of LCF life of gear shafts.