This study addressed the critical challenges of a low expansion ratio, a narrow foaming temperature window, and severe shrinkage in polyether block amide (PEBA) foams which are caused by the linear molecular structure and inadequate melt viscoelasticity. By synergistically combining epoxy-based chain extension modification (KL-E4370B) and nitrogen (N₂)-assisted gas exchange, enhanced melt viscoelasticity has been achieved through increased molecular branching (evidenced by an order of magnitude higher complex viscosity and elevated storage modulus) alongside effective shrinkage mitigation. The modified PEBA exhibits a remarkable 34.25% improvement in expansion ratio and a 65 ℃ broadening of the foaming temperature window. For the 2wt% KL-E4370B modified system, while CO₂ foaming initially produced a 22.75 expansion ratio that shrank to 5.52 (with partial recovery to 6.91), the N₂-assisted protocol achieved a peak expansion ratio of 36.58 with post-shrinkage retention at 25.64,surpassing the original CO₂ performance. Microstructural analysis reveals that the nitrogen exchange not only counteracts shrinkage through gas replacement but also promotes secondary cell growth via the formation of thinner cell walls during N₂ foaming. This dual strategy enables rapid fabrication of high-expansion, dimensionally stable PEBA foams, providing an innovative solution to persistent shrinkage issues in thermoplastic elastomer foams and significant industrial potential.