Abstract: In the actual working environment, the mechanical structure is often subjected to multi-axial non-proportional cyclic load. Compared with the multiaxial proportional cyclic load, the multiaxial non-proportional cyclic load has an additional strengthening phenomenon, resulting in a decrease in the fatigue life of the mechanical structure. With the analysis of the stress and strain variation and the failure location of thin-walled cylindrical specimen under non-proportional loading conditions, a new low cycle multiaxial fatigue life prediction model considering multiaxial non-proportional additional damage is proposed based on the critical plane approach. The new low cycle multiaxial fatigue life prediction model takes the maximum shear strain plane as the critical plane, and proposes a new additional strengthening factor, which combines the shear strain amplitude and the normal strain amplitude on the critical surface to form a new low cycle multiaxial fatigue damage parameter. This new low cycle multiaxial fatigue damage parameter not only considers the fatigue damage caused by the positive strain amplitude and the shear strain amplitude on the critical surface under non-proportional loading, but also considers the influence of the strain path change and the non-proportional loading sensitivity of the material on the fatigue life of the material. Considering the fact that the additional strengthening coefficient of the material required by the model is sometimes difficult to obtain, the approximate calculation formulas of the additional strengthing coefficient of the material is given. Only the basic mechanical parameters of the material can be used to obtain additional strengthening coefficient of the material, which is convenient for practical application of the project. The fatigue life data of eight materials were used to test the new low cycle multiaxial fatigue life prediction model. The results show that the new low cycle multiaxial fatigue life prediction model has higher life prediction accuracy than the traditional multiaxial fatigue life prediction model.