This paper proposed an improved toughness exhaustion model on the basis of the toughness exhaustion model. Specifically, in this model, a new logarithmic function of the stress amplitude is adopted into the damage equivalence equation, which can reflect the influence of load interaction. In addition, the fatigue state function using the improved model is constructed and analyzed by the probability density evolution method. Then, experimental data of four materials are used to verify the proposed model, and results show that the residual life fraction predicted by this model is accurate. Furthermore, the time-varying fatigue reliability of the proposed model is validated using experimental data of two materials. The results show that the improved model has better applicability and higher accuracy.