In the numerical simulation of rainfall intensity for flood discharge atomization, mass production of water droplets needs to be generated during the solution of random droplets splashing mathematical model, which leads to the problem of low computational efficiency. Based on the academic ideas of particle motion optimization calculation methods, such as graded droplet spectrum, representative oil droplet and particle group, the concept of graded droplets was first proposed, and a graded random splashing droplets mathematical model was established. The effectiveness of the proposed model was verified by comparing with an example with an analytical solution. The comparison results show that the solution precision of the graded random droplets splashing mathematical model with 10,000 water droplets is very close to that of the random droplets splashing mathematical model with 1 million water droplets. It is shown that the graded random droplets splashing mathematical model is more advantages in calculation than the random droplets splashing mathematical model.