In order to deeply reveal the chip formation mechanism of carbon fiber reinforced polymer (CFRP) helical milling at different fiber orientation angles (θ), for the complex oblique cutting characteristics reflected in the side edge milling process of CFRP helical milling, a microscopic finite element model of CFRP oblique cutting in matrix, fiber, and interfacial phases is constructed. The fiber failure mechanism of CFRP after machining at θ of 0°, 45°, 90°, and 135° was analyzed. The relationship between tool oblique angle (i) and chip size was explored, and the change law of residual stress with machining parameters was revealed, at the same time, the influence of the machining parameters and θ on the cutting force was elucidated. The results show that the fiber bending failure occurs at θ of 0° and 135°, shear failure occurs at 45°, and both bending failure and shear failure exist at 90°; Chip size at 0°, 45° and 90° are all positively correlated with i, and chip size at 135° is not correlated with i; Residual stress is positively correlated with cutting speed, not correlated with cutting depth; The cutting force shows the maximum value when θ=90° and the minimum value when θ=0°. When 0°<θ<90°, the cutting force tends to increase, when 90°<θ<135°, the cutting force tends to decrease. It can be seen that the cutting force is positively correlated with the cutting speed and depth, and the maximum error between the simulated and experimental cutting force values for different cutting speed and cutting depth is no more than 11% and 9%, which can predict cutting forces in CFRP helical milling.