Abstract: The continuous damage theory model fails to reflect the influence of the interaction of two adjacent load levels on the structural fatigue life under the variable amplitude loading; therefore, in this paper, considering the load order and other factors, the quadratic stress amplitude ratio is introduced into the model calculation to establishes a new load action factor, and to obtain an improved nonlinear fatigue life prediction model. According to the results obtained from the experimental tests of 45 steel and 16Mn structural materials under secondary variable amplitude load, the model is compared and analyzed with the modified continuous damage model. It can be found that the model has good accuracy and reliability and does not need to introduce additional parameters, which is more suitable for practical engineering.