Fiber bending during carbon fiber reinforced plastic (CFRP) milling is an important factor on the machined surface quality. During milling, the fiber first contacts the rake face instead of the tool edge at a certain cutting angle, and then the fiber is bent instead of being cut by the tool, causing the matrix and the fiber to fall out. The fiber is then broken from inside the machined surface and subsequently pulled out as the tool rotates, which are known as pull-out fibers. The machining defect is the main cause of deteriorating the surface quality. To reduce such machining defects, it is important to predict the fiber bending during CFRP milling. However, it is difficult to determine where fiber bending occurs because the fiber cutting angle changes every moment as the tool rotates. This study analyzed the fiber cutting mechanism considering bent fibers during CFRP milling and proposed a method to identify the type of machining mechanism through machining sound analysis. Moreover, CFRP milling simulation was performed to numerically analyze the machining parameters such as fiber cutting angle, fiber length, and the magnitude of fiber bending at different milling conditions. Through experiments, it was verified that fiber bending or defects can be identified through sound analysis of machining in the high-frequency range between 7500 and 14,800 Hz. The effect of chip thickness in up-milling and down-milling on fiber bending was investigated by analyzing simulation and sound signal, and their effects on cutting force and machining quality was experimentally verified.