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Selective laser melting (SLM) technology can realize the direct fabrication of products with complex shape, high dimensional accuracy, and excellent mechanical properties. However, improper selection of fabrication parameters of SLM will introduce defects in the products. For the problem of defects, this investigation studied the influence of the two main fabrication parameters, laser power and scanning speed, on the type, size and quantity of internal defects in fabricated titanium alloy, and explored the evolution of defects with fabrication parameters. The results shown that there are mainly irregular shape and regular spherical shape defects in the titanium alloy fabricated by SLM. The area with low laser power (≤130 W) and high scanning speed (≥900 mm/s) is mainly irregular shape defects. Insufficient energy is the main reason for formation this type defect. The area with high laser power (≥190 W), low scanning speed (≤600 mm/s) is mainly regular spherical shape defects. The gasification of alloying elements caused by excessive energy is the main reason for formation this type defect. With the increasing of laser energy density, the SLM fabricated titanium alloy processing map was drawn according to the evolution law of defects. The evolution of defects presents three stages for the area of irregular shape defects gradually decreasing, micro-scale irregular defects to micro-scale regular spherical, and regular spherical defects gradually grow up.