A modified heat-source model based on electromagnetic wave theory was proposed to investigate the interactions between powder particles and a laser beam, considering the spatial distribution of particles inside the beam. The absorption of energy by these particles in laser directed energy deposition additive manufacturing was calculated using the proposed model, which was validated experimentally. Both numerical model and experiment were used to study the effects of powder velocities on the temperature variations in the additive manufacturing process. Results indicate that the direct heat transfer from the laser to a target can be increased if the size distribution is wider; it also increases with the velocity of the particles. However, with the increase of powder-flow rate, the rate of mass transfer decreases the heat transfer. Melt-pool depth in melting and re-melting processes can therefore be controlled by varying these parameters.