Laser Directed Energy Deposition (DED) is an important process for additive manufacturing because of the advantage of high manufacturing speed. 450 nm blue laser has a higher absorption rate than traditional infrared laser for highly reflective materials such as Al and Cu. However, it is still unclear how the blue laser affects the particles and the molten pool flow in the DED process. In this work, we first designed two models to reveal the mechanism of the molten pool behaviors in the blue laser DED process: the transient model and the steady deposition model. The models are validated by in-situ and ex-situ experiments respectively. It is found that the three stages can be concluded for a particle impacting the molten pool: particle impacts, surface oscillates and the molten pool recovers. The particles whose diameter is less than 100 µm are necessary for steady deposition. The influence of process parameters on DED is investigated, and a suitable process parameters window is found. we also verified the superiority of the blue laser for DED of highly reflective materials through the simulation models. The current study provides new insights into the mechanism from the interaction in the molten pool scale to single-track forming in the blue laser DED, and can provide theoretical guidance on the application of blue laser DED of highly reflective materials.