Abstract: Carbon fiber reinforced polymer composite (CFRP) samples with different surface roughness S_a were obtained by grinding and milling (up-milling and down-milling) processes, and tensile-tension fatigue tests were carried out. The effect of machined surface roughness on fatigue performance was studied based on the stiffness degradation model. The results show that the S_a of the grinding, up-milling and down-milling samples were 1.2, 3.2, 5.9 μm, respectively. Some fibers of the 0° fiber layer of the grinding samples were missing, resulting in voids. The milling samples had many pits on the surface of the 45° fiber layer, and the pits in the down-milling samples were more and deeper. The fatigue life of the grinding samples was the highest, followed by the up-milling samples, and the down-milling samples was the lowest. With increasing S_a, the surface cracks of the CFRP samples rapidly initiated and propagated, the degradation rate of the initial stage of stiffness degradation increased, the stage I of stiffness degradation ended sooner, and the fatigue life of the samples decreased.