To reveal the potential mechanism of rainfall on the diffusion of microplastic pollution, a drop impact experiment was conducted on the microplastic particle layer. High-speed photography was used to capture the flow regime after drop impact on the microplastic particle layer, and a flow zone map was established to determine the threshold between each flow regime. The influence of characteristic parameters on the flow regime after drop impact was also analyzed. The results indicate that the flow regime of drops after impacting the layer of microplastic particles includes bouncing, spreading, coronal splashing, and prompt splashing. Increasing the Reynolds number Re₀, Weber number We₀, relative size D^* of microplastic particles, or relative thickness h^* of particle layers can promote the transition of the flow regime from bouncing to spreading and then to coronal splashing or prompt splashing. A dimensionless number M=D^*h^*We₀Re₀^-2/5 is proposed based on Buckingham π theorem for identifying the threshold between different flow regimes. The mechanism of drop impact on the splashing of microplastic particles includes forward collision and viscous carrying. The former exists in all flow states, while the latter exists in bouncing, coronal, and prompt splashing flow states. Compared with viscous carrying, forward collision driving is more likely to cause large-scale propagation of microplastic particles.