Selective laser melting (SLM) technology is widely used in the manufacture of complex lattice structures, but the thermal-mechanical behavior of lattice structures affects the forming quality of the workpiece during printing. In order to study the effect of pillar size on the temperature changes, residual stress and deformation of titanium alloy lattice thin-walled parts, three groups of Ti6Al4V lattice thin-walled parts models with different pillar sizes were designed. The temperature change distribution during the forming process was observed, and the Von-Mises stress and deformation of the formed parts after cooling to room temperature were analyzed. The results show that for lattice structures with different pillar sizes, with the increase of pillar size, the maximum instantaneous temperature increases from 1271.35 ℃ to 1396.28 ℃, and the maximum residual stress also increases from 1207.8 MPa to 1369.2 MPa. The larger residual stress is mainly distributed in the node area of each structure, and the stress in the Z-direction is the largest, followed by the X-direction and the Y-direction. Due to the increase of the pillar size, the residual stress and the weight of a single pillar increases, and the deformation of the lattice structure also increases from 0.0919 mm to 0.1730 mm, and the deformation of the pillars on both sides is observed to be the largest. Therefore, in the actual printing process, it is necessary to add support at the position with the maximum deformation according to the simulation results to prevent serious deformation of the thin-walled parts during the printing process, so as to control the forming quality.