Abstract: The second flight of the hypersonic technology vehicle 2 (HTV-2) failed on August 11, 2011. According to the engineering review board (ERB) analysis report, the most probable cause of the premature flight termination is unexpected aeroshell degradation, creating multiple upsets of increasing severity that ultimately activated the flight safety system. In order to investigate this failure, the aerothermodynamic environment, ablation, and stress are calculated and analyzed in this paper using numerical simulation and empirical calculation combined method, based on reconstructed HTV-2 configuration and the flight trajectory. It has been found that, at the height of 40km, there is a possibility of boundary layer transition, leading to turbulent flow along the leading edge of the wing. Especially, due to the shock interaction, the transition moves forward, and the heating rates are 55% higher than those with laminar flow condition at Z=200mm in spanwise direction at leading edge. The maximal value of cold wall heating rate reaches 11MW/m², and the ablation recession is approximately 3mm. Since the thickness of 2-D carbon cloth is only 1mm, there are two to three layers of carbon cloth are burned up at body leading edge. At the same time, the normal stress exceeds the bonding strength between carbon cloth layers. The most probable cause for the termination of the HTV2 second premature flight can be concluded according to the present study. The aeroshell degradation process can be described as follows. An unexpected great ablation at the leading edge breaks several layers of the carbon cloth, resulting in a long breakage, meanwhile the normal stress invalidates the bonding between carbon cloth layers.Under the influence of aerodynamic force, several layers of the carbon cloth can be lifted up from the breakage.This behaviour has a strong impact on the stable aerodynamically controlled flying, and finally activates the vehicles autonomous flight safety system to make a controlled descent and splashdown in the ocean.