Abstract: An effective triggering mechanism can induce and improve the axial progressive crushing behaviors, but the instability problem of automotive energy-absorbing structure under the oblique crushing load has not been solved. To propose new designs to improve the instability behaviors, the oblique crushing behaviors and failure mechanisms of composite energy-absorbing circular tube under the semi-circular cavity triggering mechanism were studied. The finite element model of circular tube with semi-circular cavity triggering mechanism was established, and the interface and intralaminar nonlinear damage evolution model was adopted to simulate its actual crushing failure modes. The axial crushing load, energy-absorption and failure modes corresponding to simulation and experiment were compared to validate the crushing model of circular tube. Further, the effect of oblique crushing angle on the crushing behaviors of circular tube under the semi-circular cavity triggering mechanism was predicted, and both the axial and oblique crushing failure mechanisms and their differences were revealed in detail. Results show that the crushing load, energy-absorption and failure areas obviously decrease with the increasing the angle, and the failure energy-dissipation of material is inadequate due to unstable crushing process. The circular tube under the axial crushing exhibits progressive failure, but is featured by a transition from progressive failure to instable failure for the oblique crushing, leading to a transition occurring in oblique crushing load and energy-absorption curves. This study deepens an understanding for oblique crushing failure mechanisms of circular tube under an external triggering mechanism, providing some design bases for improving instability behaviors of oblique crushing.