Abstract: During selective laser melting (SLM) process, defects will be formed inevitably inside and on the surface of the additive manufacturing component. Therefore, it is important to evaluate the impact of crack propagation at the microscale on service integrity of structures throughout their life cycle from the perspective of fitness for service. In order to quantitatively reveal the initiation behavior of fatigue cracks, the Tanaka-Mura dislocation model was used to explain the fatigue crack initiation and short crack propagation of SLM formed GH3536 alloy. The competition mechanism between the surface and internal initiation of fatigue cracks caused by defect location, defect size, and defect type was analyzed. The results indicate that as the distance from the defect to the surface decreases or the equivalent defect size increases, the fatigue failure mode changes from surface failure to internal failure. SLM formed GH3536 exhibits damage tolerance under corresponding conditions: when the defect size is small, the crack initiation mode of the additive manufacturing component remains the same; while with the rising number of defects, the fatigue short crack propagation path deflects several times and the initiation life decreases significantly.