Void defects can reduce the mechanical properties of composites and increase the potential danger of composite components in service. The effects of various porosities on damage evolution behaviors of carbon fiber/epoxy composites are deeply investigated by combining the damage visualization and acoustic emission response. The results showed that the increased porosity accelerates the damage evolution from micro-scale to macro-damage. Acoustic signals in composites are captured in advance when the porosity is high, and the voids below the kink band accelerate matrix cracking and delamination, while specimens with low porosity easily cause fiber damage and fiber/matrix debonding. Additionally, applying extra pressure during the resin transfer molding process can decrease the porosity, but excessive extra pressure may result in an inverse result. The method combining acoustic emission and micro-CT can provide a reference for improving quality and the health monitoring of composites.