This study combines experimental and numerical approaches to investigate the effects of initial debonding defects at the stringer-skin interface on the compressive behavior of composite stiffened panels. Digital Image Correlation （DIC） and strain gauge measurements were employed to characterize the buckling and post-buckling behavior. The experimental results demonstrate that while the initial debonding has limited influence on the buckling load， it induces buckling mode transition during compression and causes premature interfacial failure near the pre-existing debonded region， leading to a significant reduction （up to 18.6%） in structural load-bearing capacity. An innovative cohesive zone modeling approach was developed based on an interface failure criterion that accounts for the inhibition of shear failure due to out-of-plane compressive stress， where the intact interface and pre-debonded region were simulated using cohesive elements with a bilinear damage constitutive model and contact constitutive model， respectively. A progressive damage finite element model was subsequently established， successfully reproducing the post-buckling failure process. The numerical predictions show excellent agreement with experimental data， with buckling load errors below 4% and failure load errors within 7%， while accurately capturing the buckling modes transition and failure progression. Parametric studies were conducted to evaluate the effects of debonding size and location on the compressive post-buckling performance. The findings provide valuable references for damage-tolerant design of composite stiffened structures.