The mechanical degradation after thermal-oxidative ageing is crucial to the safety and durability design of 3D woven composites. The high-speed camera and micro-CT were employed to characterize the impact compression deformation evolution and internal damage distributions, respectively. A two-step “matrix shrinkage” method was used to study the impact compression behavior of 3D interlocked woven composites before and after ageing to reveal the degradation mechanism after thermo-oxidative ageing. The interface cracks generated on the surface of aged composites continued to increase and widen with the increase of ageing time. The impact properties of aged composites gradually decreased with increasing ageing time, and the compression modulus decreased by 16.6% after 32 days. The combined effects of thermo-oxidative degradation of resin and interface damage in aged composites hinder the transfer of stress waves from resin to yarn, leading to yarn fracture earlier and more severe damage. Meanwhile, the aged interface damage affects the expansion path of shear cracks causing wider and more severe shear bands, which severely degrades the impact properties of aged composites, but does not change the impact 45° shear damage mode. The results provide a theoretical basis for impact engineering fabrication of composites for long-term service under thermal oxygen environments.