Carbon fiber reinforced polymer (CFRP)/titanium alloy (Ti) stacks have become a research focus in aerospace and advanced manufacturing due to their superior integrated properties. However, interfacial characteristics and interlayer gaps critically influence cutting forces and hole-making quality. So, a cutting simulation model for CFRP/Ti stacks with interlayer gaps is established, the cutting force variation with interlayer gaps during milling and the mechanistic role in interfacial defect formation are deciphered through multiscale simulations. In addition, the correctness of the simulated model is verified by helical milling experiments, and the influence of machined surface quality under different clearance conditions is also evaluated. The results reveal a four-stage nonlinear evolution pattern of interfacial cutting forces during helical milling of CFRP/Ti stacks with gaps. Further investigation indicates that this nonlinear evolution pattern results in the development of localized damage evolution regions adjacent to interlayer gaps in CFRP composites, especially. A characteristic stepped distribution pattern is observed in both hole-wall topography and burr dimensions within the affected zones.