Abstract: Aming at multiaxial fatigue life prediction model research on the welding structure of aviation engine casing mounting seat under complex loads, a fatigue life prediction model based on critical plane method is proposed. Firstly, based on the structural characteristics of circular weld of the casing mounting seat, a fatigue biaxial tensile spacimen was designed, and biaxial tension fatigue tests were conducted at different load levels and biaxial ratios. Then, based on the Findely and Matake criteria of the critical plane method, the fatigue life was predicted separately, and the results showed that the two criteria could not effectively predict the fatigue life of circular weld of the casing mounting seat under different biaxial ratios. Finally, by introducing the biaxial ratio γ to adjust the fatigue life prediction model under the Matake criterion, a biaxial tensile fatigue life prediction model was established that can comprehensively consider the effects of different biaxial ratios, and the predicted results are within a 3-fold dispersion band.