Stress intensity factor (SIF) is a fundamental parameter in evaluating the fatigue crack propagation of steel structures. This paper focuses on the theoretical SIF calculation of steel plates with an inclined crack, subjected to tensile fatigue loading. Erdogan and Sih's equations for calculating the mixed-mode I/II SIF are extended to the bonded strengthening of steel plates using non-prestressed and prestressed CFRP. The prestress loss caused by the compression of the steel plate is considered. The theoretically calculated SIFs are proven to be in very good accordance with the numerical simulation. Due to the fact that all mixed-mode fatigue cracks develop soon to the tensile predominant mode under tensile fatigue loading, a simplified method is proposed for fatigue life estimation, in which the cracks propagate under mixed-mode in the very beginning and then perpendicular to the loading direction. The simplified method shows a fairly good capability of the fatigue life estimation.