Abstract: In this paper, based on experiment, corrosion crack extension mechanism in injecting water pipeline where corrosion is main and fatigue is second is studied, namely corrosion fatigue crack extension is mainly caused by passive film cracking. On the basis of the mechanism, mathematical model of corrosion fatigue crack extension velocity is established and effective methods of solving the model are gained. Take the given injecting water pipeline that is added into inhibition corrosion for YING-11 experimental zones of Shengli Oilfield as an example, the corrosion fatigue crack extension mathematical model established in this article is used to predict corrosion fatigue defect sizes varying propensity with time, and is verified that prediction values are good agreement with field test value, which shows that the mathematical model established in this paper is valid right.