Titanium (Ti) and its nanoparticles are widely used in 3D printing due to a high corrosion resistance, biocompatibility, and lightweight potential. However, the atomic-scale mechanism of sintering densification and phase transition remains unclear, which limits the performance regulation of additively manufactured Ti components. In this work, molecular dynamics simulations were performed to study the sintering and phase-transition behavior of multi-sized Ti nanoparticles under 3D printing conditions. The melting point of 2 nm Ti nanoparticles decreases to about 1000 K, while that of 12 nm particles approaches the bulk value of 1941 K. Two distinct stages were observed during surface-energy-induced phase transition: initial lattice reconstruction at 800–1100 K, followed by complete melting above 1500 K. Smaller nanoparticles exhibited stronger pre-melting and faster atomic diffusion, which effectively filled interparticle pores and reduced the porosity. Increasing particle contact points shortens atomic diffusion paths, accelerates densification, and alleviates stress concentration. This study reveals the size-dependent sintering mechanism of Ti nanoparticles at the atomic level, providing theoretical guidance for the process optimization, microstructure control, and performance improvement of aerospace and biomedical titanium components fabricated by additive manufacturing.