In order to research the migration and transportation of microplastics emitted from China's coastal waters, we collected data from coastal fisheries and tourism industries, calculated the mass of plastic waste entering the sea from four sources: marine fish aquaculture, coastal tourism, rivers, and nearshore fishing. Then, we converted it into the mass of microplastics entering the sea at a certain proportion, and discretized the spatiotemporal distribution of microplastics based on this, providing numerical simulation with the setting of source emissions and specific discharge locations. Based on the Lagrange random walk model, a three-dimensional physical migration numerical model of microplastics in the ocean was constructed to investigate the transport and migration patterns of four types of microplastics discharged from the Chinese coastal areas in different water layers of the Northwest Pacific. The four types of microplastics include: density of 1010kg/m³ with a particle size of 0.1mm, density of 1025kg/m³ with a particle size of 0.1mm, density of 1025kg/m³ with a particle size of 0.6mm, and density of 1040kg/m³ with a particle size of 0.1mm. The simulation results show that the distribution and fate of microplastics with different densities are different. Microplastic particles with a density of 1010kg/m³ and a particle size of 0.1mm follow the ocean current for a long time, and eventually many particles land on the shore, while some still remain on the sea surface and continue to float; microplastic particles with a density of 1025kg/m³ and a particle size of 0.1mm, due to their higher density than nearshore seawater, a large portion of the particles will sink to the bottom. At the same time, after being released, a large portion of the particles will go ashore under the reciprocating motion of nearshore currents; microplastic particles with a density of 1025kg/m³ and a particle size of 0.6mm, due to their higher density than nearshore seawater and larger particle size, have a significantly higher proportion of sinking compared to the  case with a density of 1025kg/m³ and the 0.1mm particle size . Similarly, a small portion of the particles are released and go ashore under the reciprocating motion of nearshore currents; microplastic particles with a density of 1040kg/m³ and a particle size of 0.1mm, due to their higher density than nearshore seawater, more than half of the particles are deposited on the seabed.
 

microplastic;three-dimensional;numerical simulation;migration;density