Prediction of fatigue failure behaviors by simplified uniaxial loads are no longer applicable since many of the engineering structures are often subjected to complex multiaxial fatigue loads during service. Therefore, accurately predicting the multiaxial fatigue failure behavior of engineering structures under complex loads is of great significance for improving structural safety. Fatigue crack initiation and propagation is the most intuitive response to fatigue failure behavior. In this study, multiaxial fatigue tests under different stress amplitude ratios with fixed von Mises stress amplitude were carried out by using 2A12-T4 aluminum alloy solid round bar specimens. The surface crack initiation and propagation path of the specimen under different stress amplitude ratios were studied by observing the specimens under a metallographic microscope, and the multiaxial fatigue failure behavior of 2A12-T4 aluminum alloy under different stress ratios was discussed. Results show that many cracks initiate on the surface of all the specimens, but the main crack which causing fatigue failure was only one. The crack initiation direction is always close to the maximum shear stress amplitude plane, and the length and direction of the stage Ⅰ crack are all affected by the stress amplitude ratio. Propagation of the main crack are mainly along the maximum shear stress amplitude plane, which means the maximum shear stress amplitude is the main control parameter that causes the multiaxial fatigue failure of 2A12-T4 aluminum alloy.