The tensile damage evolution of 2D plain woven C/SiC composites strengthened with 1K and 3K carbon fiber bundles and microstructure’s influence on material’s damage evolution were investigated using the Acoustic Emission technology (AE) and failure observation. Experimental results reveal that damage evolution of these two kinds of composites is a gradual procedure and this procedure consists of three phases. There is no damage during the first phase. During the second phase, the damage, mainly consisting of matrix microcrack cracking, interface debonding of fiber and joining of microcrack, random takes place in the whole area of specimen. During the third damage phase, the damage, mainly consisting of macrocrack cracking, fibers breaking and fibers pulling out, mainly takes place in the local failure area of specimen. Because the microstructures of composites with 1K and 3K carbon fiber bundles are different, their damage mechanisms are different. Composite strengthened with 1K carbon fiber bundles get in second phase at 90% failure stress, and their main energy dissipation occurred during the second damage phase. While Composite strengthened with 3K carbon fiber bundles get in second phase at 80% failure stress, and their main energy dissipation occurred during the third damage phase.