Abstract: The low-frequency fatigue behavior of 20Cr2Ni4 steel at various stress ratios(including SCCR=1)in aqueous environments is investigated. It is discovered that the fatigue threshold(△K_th) decreases and crack growth rate(da/dN)increases with increasing stress ratios.As the pH values increase, the △K_th decrease slightly but the da/dN at the plateauregion increases greatly. The effect of environments on crack growth relates to the strain and strain rate at crack tip. The concepts of activation factor(A) and critical activation factor(Ac)and the calculating equations are proposed.