The dynamics model of plume for underwater oil spill is improved by considering the drag force of the plume. The comparisons between prediction and experiment show that the improved model is viable. The improved model combining the Lagrangian random walk method is used to simulate the entire oil transport process from release exit to water surface in underwater oil horizontal release and some new results are obtained. Firstly, the release rate, the current velocity and its direction have evident influence on plume dynamics. The plume length increases as release flow rate increases. For small oil release rate, the plume length is very short so that the plume stage can be ignored due to the small initial momentum. When the oil release in direction is the same as the current, the current will play a positive role in the plume motion and the plume will therefore extend a longer distance. In this case, the greater the current velocity is, the longer the plume extension length will be. And when the oil re-lease in direction is opposite to the current, the plume first moves forward and then is reversed to move along the opposite direction. The position of the reversal point depends on the current velocity and the release rate. Sec-ondly, the release rate and the current velocity also have a significant effect on oil droplet diffusion. For small oil release rate, the oil distribution area is narrow and the concentration near upper side of droplets moving path is relatively higher. As the release rate or current velocity increases, the oil droplet diffusion area expands away from the release point. Finally, the movement characteristics of oil droplets in the plume and diffusion stages will affect the time and location for oil droplets reaching the surface. In a small release rate as well as a small current velocity, the time for oil droplet reaching surface is very short and the horizontal position reaching surface is closer to release point. In this case, the time spent in the plume stage is extremely short compared to the time spent in the entire oil motion process. Therefore, the plume dynamics stage can be ignored in this case. With the increase of release rate or current velocity, the plume will be extended and the time spent in plume dynamics stage will also be longer, so the plume dynamics stage can no longer be ignored for larger release rate or current velocity. In the case of reverse current, as the plume momentum be partially consumed by the reverse current, the plume is reversed and decays quickly, the time taken in the plume dynamics stage is relatively shorter than that for positive current. Correspondingly, the total time is also somewhat shortened.