Abstract: The charcteristic parameters of defects in electron beam fusion additive manufactured Ti-6Al-4V alloy were obtained by high-resolution synchrotron radiation 3D X-ray tomography technique. Combining with high cycle fatigue tests, the influence of defects on fatigue properties of the alloy was analyzed. The Z-parameter fatigue life model was modified by considering defect characteristic parameters, and an X-parameter fatigue life model was proposed. The results show that there were porosity and incomplete fusion defects in the manufactured Ti-6Al-4V alloy. The fatigue life of the alloy had significant dispersion in different stress levels and the fatigue limit with 50% survival rate was 679 MPa. Most of the fatigue cracks originated at the porosity and incomplete fusion defects. The linear fitting correlation coefficient of X-parameter-fatigue life curve by X-parameter fatigue life model was 0.878 5, and was improved by nearly 20% compared with the Z-parameter fatigue life model, the dispersion of fatigue life significantly decreased, and the prediction accuracy of the model was significantly improved.