Abstract: To reveal the effect of particle-size distribution on the sintering behavior and densification of metal-injection-molded Ti–6Al–4V alloys, the polycrystalline multi-particle and multi-size molecular dynamics models were established. Two sintering systems with wide particle-size distribution (M1, 5～12 nm) and narrow particle-size distribution (M2, 7～9 nm) were constructed. Molecular dynamics simulations were performed in the NPT ensemble using the embedded-atom method (EAM) potential. Particle evolution, sintering density, atomic diffusion, and defect evolution were analyzed, and the metal injection molding sintering experiments were carried out to verify the simulation results. The results show that the melting temperature of Ti–6Al–4V nanoparticles exhibits an obvious size-dependent effect. The sintering mechanisms include particle contact driven by surface energy, temperature-accelerated surface diffusion, micro-plastic flow of small particles, and densification driven by stress-dislocation synergy. In the wide-size-distribution M1 system, the atoms in small particles possess the higher activity, leading to higher pore-filling efficiency and higher final sintering density compared with the narrow-size-distribution M2 system. Gas-atomized Ti–6Al–4V powders with broad particle-size distribution were adopted for the metal injection molding experiments. The sintered samples achieve the relative density of 99.1%, tensile strength of (1033±12) MPa, and yield strength of (965±12) MPa. The electron back scatter diffraction characterizations confirm that the phase transformation preferentially occurs at grain boundaries, and the dislocation tangles and pile-ups are concentrated at grain boundaries and phase-transition regions, which are consistent with molecular dynamics simulation. The reasonable regulation of powder particle-size distribution can effectively promote the sintering densification of Ti–6Al–4V alloys.