An irregular porous structure of Voronoi polygon was constructed by Rhino software, and the porous TC4 titanium alloy was prepared by selective laser melting (SLM). The effects of laser power (180, 200, 220 W), scanning speed (1 200, 1 600, 2 000 mm·s-1) and scanning spacing (80, 100, 120 μm) on the alloy microstructure were investigated. The results show that with the increase of laser power, the decrease of scanning speed or the increase of scanning spacing, the number and size of micropore defects in solid part of SLM formed porous TC4 titanium alloy decreased, and the relative density increased; the scanning speed was the main reason affecting the formation of defects. Under laser power of 220 W, scanning speed of 1 200 mm·s-1 and scanning spacing of 120 μm, the titanium alloy had the fewest micropore defects with relative density of 99.2%. There were equiaxed crystals and columnar crystals parallel to the substrate plate surface in the section near the pores of the porous structure, while the microstructure far away from the pores was mainly composed of β columnar crystals, and inside the columnar crystals there was primary acicular martensite arranged in parallel at ±45° to its major axis. With the decrease of laser power, the increase of scanning speed or the decrease of scanning spacing, the width of β columnar crystal and the length of primary martensite decreased; the scanning spacing had a greater effect on the microstructure.