Molecular dynamics (MD) simulations were performed to research the miscibility, glass transition temperature (T-g) and mechanical properties of nitrocellulose/dibutyl phthalate (NC/DBP) binary systems. The solubility parameters () of NC and DBP were calculated to predict the miscibility. NC and DBP are miscible as a result of a small (<2.0MPa(0.5)) between the two components. The free volumes (V-F) and density () of NC/DBP system were simulated to study the T-g. It is found that the V-F and of NC/DBP changes regularly with the increase of DBP mass fraction and the transition occurs at the turning point. The simulation results demonstrate that DBP makes significant contribution to the reduction of the T-g of NC. Additionally, the mechanical properties of NC/DBP systems including Young's moduli (E), Bulk moduli (K), Shear moduli (G) and Poisson's ratio (v) were calculated. The results reveal that with the increase of DBP mass fraction the ductility of NC/DBP improves while the brittleness decreases. Overall, a new and systematical method is proposed to study the miscibility, T-g and mechanical properties of polymers.