Abstract: For the study of the damage evolution and the effect of fiber pre-broken on the mechanical properties of composites for wind turbine blades, unidirectional and loading-unloading tensile tests of unidirectional and multi-axial composite were conducted. In this experiment, real-time acoustic emission monitoring was used, and tensile mechanical properties, damage failure characteristics and the corresponding acoustic emission response characteristics were obtained. The results show that because of the presence of fiber pre-broken, the damage of matrix and interface in the defect location and the adjacent region of unidirectional composites appears obviously when the load reaches to about 30% of the failure load. As the load increases to about 60%, interlaminar shear failure between defects layer and adjacent layers are induced, leading to a sharp reduction in stiffness, the acoustic emission cumulative hits are significantly higher than that of the defect-free ones. As the load reaches to about 60% of the failure load, the obvious damage of matrix resin in fiber pre-broken region occurs in the case of multi-axial composite with fiber pre-broken. As the relative stress level increases, the Felicity ratio of multi-axial composite gradually decreases.