Abstract: The fatigue crack propagation (FCP) behavior and mechanism of pearlite-ferrite casing-drilling steel was investigated by means of fatigue tester and scanning electron microscope. The results show that stress ratio has not a remarkable influence on Paris parameter m of the pearlite-ferrite steel, but has notably influences on Paris parameter C. The effective stress strength factor range ΔKeff is the dominant material parameter which governs the FCP rates. At the modified Paris linear zone, the stress ratio does not notably influence on the FCP rates, both meff and Ceff are constants. The FCP threshold values exhibit a significant decrease with the increase of stress ratio, which can be attributed to the crack closure effect. At the higher stress ratio, the plastic deformation is not prone to occur on the fracture surface of the threshold zone.