To verify the effectiveness of a novel axisymmetric scramjet, Improved Delay Detached Eddy Simulation (IDDES) coupled with Dynamic Zone Flamelet Model (DZFM) based on 60 million cells was conducted to investigate the performance of the full-scale engine. Four mesh sets with different refinement levels and adopting hexahedral structured cells were used for the grid independence verification. The pressure agrees well with the experimental data. Due to the absence of the corner effect, the boundary layer is thinner and the jet penetration depth is low. The poor mixing between the transverse fuel jet and the crossflow causes weak combustion and a low-pressure rise ratio. The cavity plays four roles in enhancing combustion: a radial pool, a low-speed bay, a high-temperature zone, and a premixer. Axisymmetric scramjets without any flame holders overall have poor mixing and combustion performance.