By changing the forming parameters and heat treatment temperature, the forming quality of selective laser melting (SLM) TC4 titanium alloy was optimized. The results indicated that the relative density of the TC4 sample could exceed 99.90% under the given process parameters. In particular, the highest relative density, 99.993%, was obtained with a layer thickness, laser power, scan speed, and hatch spacing of 40 μm, 200 W, 1500 mm/s, and 0.065 mm, respectively. Furthermore, the corresponding tensile strength reached 1149 MPa and 1111 MPa and the relative plasticity was as low as 8.1% and 5.1% for X/Y and Z directions, respectively, owing to the existence of a large amount of martensite α' phase inside. Moreover, the martensite α' phase gradually decomposed to the (α+β) phase through heat treatment. Therefore, the tensile strength gradually decreased as the plasticity increased when the heat treatment temperature increased from 650 °C to 950 °C. After heat treatment at 950 °C, the tensile strength of the SLM TC4 samples decreased to 78.9% and 80.5% of that before heat treatment, while the elongation increased by 103.7% and 152.9% for the X/Y and Z directions, respectively.