A ducted fuel injection (DFI) strategy has been proposed as an efficient approach to reduce the soot emission in direct-injection compression ignition engines. By injecting the fuel through a small tube within the combustion chamber, a leaner air-fuel mixture is generated compared to the conventional free spray approach, which significantly inhibits the soot formation and helps to reduce the dependence of the engine on after-treatment systems. However, the soot reduction mechanism is still not fully understood. Therefore, in this work, a three-dimensional computational investigation was performed to explain the experimental results. Four different reduced chemical mechanisms were used to simulate the reacting spray A (n-dodecane) data from both the Engine Combustion Network group and literature. An improved post-processing method was also proposed to investigate the detailed combustion feature. The results revealed that the ignition processes using different mechanisms were all dominated by the same reaction CH2O + OH = HCO + H2O. Of the four reduced mechanisms, Yao mech demonstrated the best-predicted performance. Compared to the free-spray case, the DFI case generated a longer ignition delay and lift-off length and lower soot concentration owing to the significant reduction of air entrainment and longer core jet velocity from the duct exit to the lift-off length location. In addition, the DFI case had a significantly longer low-temperature heat release region but a shorter high-temperature heat release region and a smaller core between these two regions, which helps to reduce the sooting tendency.