With a validated fluid-structure-fracture coupling approach, this paper studied the dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation. Fracture of tube was judged by a bivariate failure criterion derived from the underlying failure mechanism at high strain rate conditions. A programed burn approach based on the CJ theory was applied to simulate gaseous detonation. The coupling between detonation wave and tube was realized by penalty contact algorithm with an improved contact stiffness calculation formula. It was demonstrated that the peak pressure at tube edge is 29% higher than that at the middle of tube face. The dominant crack driving force comes from the specific vibration and deformation modes of square tube, where the deformed round section of tube corresponds to the maximum stress wave that travels behind the flexural waves on the tube. Above mechanism makes the backward cracks branch or turn before the forward cracks and the speeds of front and back branch cracks comparable to each other, which is opposite or different from the cases of round tubes. The crack behaviors with different initial flaw locations and detonation pressures were summarized and identified in detail. The forward crack speed can be up to 900 m/s, while the backward crack speeds are generally 65%-85% of above and the branch cracks run at about 100 m/s. In addition, the crack speed has a certain increase immediately after crack branching or turning. Among the three initial flaw location cases, the tube with initial flaw at the middle of face is most resistant to crack propagation under internal detonations. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.