Abstract: The influences of different stess ratios R(0.1, 0.3, 0.5, 0.7) on the corrosion fatigue behaviour of S135 drill pipe steel in the weak alkali medium (3.0wt% KCl solution) were investigated by the fatigue tester and scanning electron microscope, and then the corrosion fatigue crack propagation rate expression and the relationship of R vs the stress intensity factor range ΔK at the fatigue crack critical tearing point were obtained. The results show that with the increase of R, the ΔK at the crack critical and final tearing points of the specimen decreased gradually (the relationship between R and the critical tearing point ΔK was obviously linear); the corrosion fatigue crack propagation rate also increased while corrosion fatigue relative life decreased and the crack mechanism changed from stripe to cleavage fracture.